I think it could go both ways.. It may help some learn to be more comfortable talking with other people but personally I feel like games kind of took away the last of my social skills (which wasn't much to begin with, hah). I'm not exactly blaming the games themselves, I'm focusing on the isolation they can possibly lead to. When I think of my childhood I don't have any memories to look back on where I was having fun hanging out with friends. I can only remember the endless hours I spent in an MMO. Even if I didn't play games though, like someone else said, I would very easily be able to find another solitary distraction.
I spent so much time stuck in front of my computer that I kind of forgot how to act & talk to people in real life. I would sit in school daydreaming about going home as soon as possible so I could level up, lol. And its funny, I have SA even online, so I don't really interact too often with other people in game. On one hand it sort of doesn't bother me because I'm naturally an introvert & enjoy time by myself. But, once in a while, I do get lonely and wish I actually had friends in real life, or at least still had some social skills left to make some, hah.