Oh I can field this one :D (I'm doing my thesis on introversion and extraversion).

Basically social situations drain introverts. You've probably got at least one friend that bounces out of (larger) social situations fairly early, or is spacing out most of the time, or is otherwise finding a way to do their own thing while it seems like most everyone else is having the time of their lives. They're most likely introverts. They process internally and have a different type of social energy than extraverts.

A book I once read put it as "extraverts are like motels; they have room for everyone and people can pop in and out pretty much whenever they want," the 'iconic' example of this is the social butterfly who seems to know everyone and is always making small talk and flitting on after the fact. Introverts, on the other hand, "are like five star luxury hotels, where only the VIPs get to stay and are given extremely personalized treatment." Those would be the friends you have who can stay up till two in the morning discussing your passions with you, or who you can talk to about anything as long as it's something important.

Now, to focus back on the issue at hand, introverts only have so much social energy to expend and therefore tend to need to "recharge" after social situations which may leave them feeling drained. These are usually your "curl up with a book, a cup of [insert hot liquid here], and a warm fireplace" types, or your gamers who spend more time exploring single player games than you could even think of logging on a multiplayer game.

TL;DR Too much social = a drained and cranky introvert with a need to recharge.