For me, Facebook started off as a way to stay in touch with friends. Gradually I made new friends(YEs Facebook friends!) and reconnected with old friends. It's no news how easy it is to build a network on facebook.
Relationships have been formed, and some broken (through facebook)...
I just recently realised the power of advertising ;) You start a group, invite your friends to join and they in turn invite their friends and so on. If you are not a fan already, here's a link to Alexis Mag-Admot's facebook page.
How have you used your facebook?