Is that list for actually socializing or building links? I am not much fan of social networks but I did catch a webinar on social media was either from CAP or IGB.
I personally use digg and stumbleupon, and sometimes delicious. Reddit is another big one, but I have logged like once.
The rest are g+, facebook and twitter that again I use for link building and not really to participate social activities. They did recommend participating, but I can’t imagine spending 3 hours in such sites to talk to people, when I’ve got work to do.