My usual go to sites for Social Networking these days are all over the map.
Twitter - a good place to see a steady stream of consciousness and an occasional cool link.
Facebook - my first step into SN. Beyong playing Scrabulous and Oregon Trail with friends and strangers, I am not the originator of much communication through this medium.
LinkedIn - a business Facebook with the centerpiece around my resume.
Inman - a huge resource and a great way to stay in touch with people in the industry.
Trulia Voices - I get a steady stream of questions and answers (including mine) that pertain to topics I am interested in. The alternative to blogging by real estate agents.