I've never really gotten into social networking sites very deeply. I used to joke that the Frienster model was "fuck your friend's friends (because you can't fuck your friends)". I use LinkedIn for professional networking, and I setup a Myspace page to see what is was about but never really got into it. It seemed to skew a bit young for me and I don't troll the singles scene.
I created a Facebook page a few months back and am pretty happy with the space it occupies somewhere between LinkedIn and MySpace. I've also started to explore the many applications that are available for it, very cool.
The group feature also has a lot of potential. I've joined several like the AIGA and MyHome 2.0. I am a charter member of Ad Bloggers.
Would love to hear any perspectives on Facebook and about any applications that people are particulary pleased with.