I like to think that I'm really open online, mainly on IRC, which I really consider to be a place to just relax and talk a bit about random stuff. I don't have a problem giving people personal information at all, except for stuff like phone numbers. But that's pretty much the only thing I'm not willing to share. I kinda share TPX's vision: it's very likely I won't meet any of you in real life anyway, so I don't have a problem with telling you a lot about myself, things I may not even tell my real life friends. A phone number just crosses the line a bit IMO. But like I said, I'm pretty sure I'll tell you just about everything else you want to know.
As for why I am so open, there are two reasons I think that play a role:
1) What TPX said
2) The amount of friends I have in real life is somewhat limited, mainly due to the fact I don't share many of their interests, so if they plan to do something together, it's not very rare for me to stay behind (although this has resulted into me getting left behind when they were going to do fun stuff as well...). The thing is, no one in my class or school even seems to like Pokemon (and yes, I do know that for sure because my school is really pretty small (not more than 600 students at max)). In here, everyone at least takes interest in pokemon, which already gives us a common hobby. On top of that, the internet is mainly based upon talking to each other: there's no going out, no cinema, no disco, pretty much every part of a "friendship" over the internet is based on talking to each other and that's something I don't have a problem with at all.