I'm not a strong user of social media sites. Actually, wait...I use Linkedin, Flickr and delicious, and of course wiki and YouTube. I also read select things on Twitter and do use stumble.
What I don't use is Facebook. Not interested, thanks. But, even if I'm not a user, it's worth paying attention: "With more than 100 million users each, MySpace and Facebook..."
Considerable work is currently being done in Social Media Marketing. It is incumbent on marketers to keep tabs (pun intended).
Oh, and be careful what you do out there. The man is watching.
UDPATE: Kids are doing this more than ever. "According to a recent study, more than 750,000 kids between the ages of 8 and 12 have set up a profile on the big social-networking sites." A disturbing, emerging trend.
UPDATE #2: Porn is no longer king. Social Media sites are...wha?