Do I have to say anything more?
Every bloody person I know is on Facebook now. And since Facebook started opening up to 3rd party sites like Twitter, Del.icio.us, Upcoming, Flickr, Last.FM, etc there's even more of a reason to use it. The weird thing about facebook for me is how odd a mix it is of web 1.0 and web 2.0. For example its got the community and participation stuff down really well but its interface and experience is franky dirty and feels like it belongs in the internet archive.
I heard a interview a while back talking about how facebook was going to replace the operating system on your machine. Although I laughed, I can see what they mean now. I imagine its the starting destination for a lot of people, and with its mixture of chat, photos, groups, etc all blended together. I guess this what Yahoo tried to do with Yahoo 360 somewhat failed?