social networking

Seems like there are dozens, maybe hundreds of web-based social networking web sites out there. Which ones do you use? I am registered on the following, though I only look at a few of them regularly :

• myspace.com
• linkedin.com
• classmates.com
• reunion.com
• graduates.com
• facebook.com
• plaxo.com
• millcreekalumni.org (only useful for McDowell alumni)

Are there any others that you find particularly useful?
[This is a revised post. Original is found on classmates.com.]