Which Social Networks, and Why?

I make daily use of Facebook, Twitter and Foursquare. 

Facebook keeps me in touch with so many old friends I’d otherwise lose track of entirely.  Facebook makes it easy to see and comment on the photos and updates we share, and to remain connected.  Like many, I worry some about privacy in the face of changing Facebook tools and policies, and the complexities of rights granted to applications that work together with Facebook and with external facilities.  But all things considered so far, Facebook does so much for me that I count myself as a fan.

I use Twitter to get news and information from my favorite sources, and to send shorter updates.  I also promote this blog using Twitter.

I use Foursquare to study location services, and I use it as sort of a perpetual game of point and badge collection.  I wrote about my Foursquare experience in a blog back on May 6th.

I've been experimenting with a workplace social network called Rypple to coordinate goals and provide online recognition with other staff.  So far, it seems promising.

I have accounts on numerous other social networks, and ignore most of them most of the time.  This may be because I don’t entirely understand their intent and their potential value to me.  Today's blog is really a set of questions for you.  Please tell me which of the other social network tools you use and a few words on why I should consider or reconsider them.

Plaxo, LinkedIn, Xing, Classmates, Jaiku, Ping, Last.fm, Instagr.am, Stickam, Tripit, Tumblr, just to name a few.  

• Do you use some of these?  Do you have other favorites?
• How do you use them, and what advantages have they brought you?
• Are some of the photo sharing apps "social networks" in your mind? 
• Is Skype a "social network"?

Please post here to tell us about any social networks other than Facebook and Twitter that you use and really like.