Nobody plays for just one reason.
I probably play for quite a few.
I'd say one of the major reasons, which has already been said, is "I wouldn't say for me it is about escaping reality, but it certainly is interesting to peer in to the realities that other people have constructed". It's always great imagining what it would be like in a different type of reality. It'd be quite a different world inside of a videogame. One game that I was practically addicted to was the Fear 2 campaign. I would wait till midnight or so, grab my nice headphones and then play with the volume up pretty high. After like half an hour or so, I basically thought I was in the game. I almost pissed myself when I heard some weird noise, let alone when something jumped out at you.
That's just the best example of how I play to escape reality.
Other then that, I play to waste time, or when I'm asked by my friends to play.
Social gaming is great because I can keep connected with my friends and family even though we're all across the east coast. I'd say that's one of my major reasons to play too. I'd much rather be playing Gears 3 with my friends then be on the phone, or facebook talking to them. Because, lets face it, you can do anything you want through games nowadays, the same can't be said about or lives.