Creating social encounters

Consider your group

The first thing you'll want to do is consider the group you're playing with. Social encounters tend to work best with people who role play their characters, but it can work with those who don't as well. In the case of the latter it does usually require them to at least describe what their characters do and say in greater detail, but it can work without it as well. This depends entirely on how you handle it however. Whenever the players give you very little to work with it can immediately become nerve wrecking or difficult to continue.

But assuming you have a group who enjoys a social encounter, and is willing to either role play or describe in detail, you'll still have to consider what your group is made up of. What their strengths are, their weaknesses, their character flaws, and so on. Just like in a combat encounter you ideally want to give all characters an opportunity to shine, no matter whether those opportunities actually come to fruition.
It's no fun to watch the most charismatic character of the group do all the talking to ensure the best chances of success after all, so try to give other players a chance to use the strengths of their characters. Perhaps the hulking physique of the barbarian is charming one of the guests, maybe that laser gun held by the engineer is catching the attention of a professor of intergalactic weaponry, and so on.

There are many similarities between creating a social encounter and a combat encounter, as you'll see in some of my next points. But realizing this can help a lot with planning a social encounter when you're used to creating combat ones.

Preparation

But before we delve into the similarities and other elements of the different encounters I first want to point out that preparation is definitely important in the case of a social encounter. Improv is definitely a possibility to some extent, and it'll definitely be part of it in many cases, but improvising the entire encounter is asking for plot holes, inconsistencies, and other elements you'll want to avoid.

You don't need to do a massive amount of preparation though. All you really need to know is what information needs to be uncovered, which non-player characters (NPCs) have this information, what their goals, flaws, traits, etc. are, which NPCs are trying to prevent this information from being uncovered, and what their goals, flaws, etc. are.
Besides that it would be wise to have a list of random characters (plus their traits, etc.) to draw from at any given time. If the party ends up talking to a random, unimportant NPC out of the blue you'll want to be able to play it off like they're part of that universe, and not just some background filler, as technically they're both.

Simply having character archetypes ready will help, as then it won't matter if they talk to a cook, a pilot, or the last person to deliver mail, you simply give whichever one they talk to a random personality from your list.

Location

So, back to the similarities of the encounters. Just as location is important in a combat encounter, so too is location important in a social encounter. You could call it the battlefield in both cases, but one is a fight of weapons, the other is a fight of wits.
But besides the obvious 'this encounter takes place in this location', you'll also want to think about where the important NPCs are in said location, as well as any parts of the location that are off limits.
If the party has to try to convince a ruler to strengthen the borders while at that ruler's dinner party, they're probably not allowed to venture into all the wings of the mansion. Neither would some of the staff members be allowed in all the sections of the mansion, but breaking these rules might be required to get the information the party is after.

The location itself could also pose threats to the party. Surveillance cameras, security dogs, tall fences, suspicious NPCS, and so on could all require the party to either think differently or simply deal with these problems head on.

Goal of the encounter

As mentioned earlier, you need to know what information needs to be uncovered in a social encounter, but this doesn't necessarily have to be a secret. It could simply be trying to let a ruler know of an impending threat while proving you're not a spy, or proving the party's innocence in a trial, or perhaps testing the party's wits and worth in verbal combat with a distrustful leader.
Part of all this is knowing what the goal of the players is, and what the goals of the NPCs are. These two elements will be the foundations of the entire encounter, as these will not only determine the difficulty of the encounter, but also the way it's fought.

"Weapons"

Just as the location could be seen as the battlefield, so too can the means of overcoming a social encounter be seen as weapons. Flirting, threats, alliances, alcohol, taking advantage of character flaws, and even torture could be seen as weapons of the social battlefield. Which weapons you provide your group with will depend on both the group, and the NPCs of course. It's definitely worth to think about this while planning the encounter though. Which NPCs would spill the beans under pressure? Which one would do so when threatening their loved ones, their pride, or their status? Which NPCs would start blabbering once they've had too much to drink?

These elements not only help with providing the group multiple ways of solving the encounter, not that they wouldn't come up with more on their own, but it also helps with creating a varied range of social encounters throughout your campaign, which can be just as important.

Diplomacy

I briefly mentioned alliances as a weapon, but alliances go a step further than simply providing leverage for the group. Diplomacy could mean specific ways of speaking or presenting yourself, it could mean avoiding throwing insults at a specific party, it could mean diplomatic immunity, and much, much more.
But with diplomacy there's often also conflicts of power. One side may wish to see the other fall, some sides may wish to aid another for their own benefit, and some sides simply wish total domination. Having two sides is usually easy, but you could make a social encounter far more interesting when there are more sides to deal with, perhaps even to the point of casuing a divide in your party.

Making friends with one side often also means making enemies with the other, so befriending an NPC, even if it's purely for their information, could have dire consequences when dealing with some of the other NPCs. Either way there's usually no way to please both sides, so choices will have to be made, choices that could have world affecting consequences.

Stakes

In a combat encounter the stakes are life and death, but in a social encounter the stakes may not always be as obvious. Sure, missing out on some information might be a hassle, but there's plenty of other quests, right? Still, you can raise the stakes immensely in a social encounter, and it, if played out well, can make for a far more exciting encounter than a combat encounter.

The stakes can be raised easily. An imminent threat that would succeed if the party fails, losing out on a chance to save somebody, world affecting events, even the threat of combat can be enough. But it can also be done by simply making the existing elements at play be present more strongly. That villain the party's been after could be at the party, and openly mock them while under the security of the guards, diplomatic immunity, or simply a lack of evidence. That NPC the party's been talking to and gaining intel from could turn out to be a spy for the wrong side who has worked against them this whole time. Finding out information one way could mean having to break a friendship with a beloved NPC. Some decisions that need to be made could even put the lives of NPCs at risk, or change them for the worse. There's a whole lot to play around with, but it does take knowing your group a little to figure out which stakes work the best. Some groups might not care for the fate of NPCs for example, while others might gladly resolve everything with combat.

Rewards

With stakes come rewards of course, but some might wonder what kind of rewards to give. There's (ideally) no dead body at the end of the encounter, which can be pillaged for loot. There's no tally of xp for each defeated monster. So where are the rewards? Well, you can still give the party xp based on the amount of people they've managed to gain information from, or based on how many pieces of crucial information they received. Doing so will also show that resolving combat through communication can be an option in the future.

But you can easily give loot as well. One grateful NPC might give the party an expensive bottle of wine for standing up for them, a guard may give up their enchanted weapon after realising they no longer wish to fight for a certain cause, and there's always an opportunity to hide some chests in a spare room. Of course looting those chests could have repercussions in the future as well, wink wink, nudge nudge.

Avoiding combat

Lastly, some of you may prefer it if combat was avoided all together, or at the very least if the option of combat wasn't as appealing to the party. This is where the stakes come in handy. Avoiding combat is as easy as making the consequence of combat something the party wants to avoid. Imprisonment, the loss of a friend, the ruining of their reputation, the abandonment of previous allies, the end of a quest. There's countless ways in which combat could result in a horrible mess the party wants to avoid. Of course when one door closes, others often open.