I don't get invited out much, but there are times when I do that I would still rather stay in and read, or play computer games, not that I am antisocial or don't like to interact with my friends, but I prefer the relative quiet where I can talk with people, catch up and stuff, not loud music of clubs and parties. Few quiet drinks in a pub then see a film, sure, night on the town and then off to a club or party, nah, I'll stay in.
Come to think of it, it could be the fact I've turned down so many that I don't get invited out more.