I only use Trulia for interacting with others about real estate. I use realtor.com to search for real estate outside of my ls service. I use craigslist to search for real estate for rent. I found active rain to be worthless unless your going to fork over the money to make your blog available for the public to view it. Other sites i gove notable mention to that i visit once in awhile are:
sjrates.com for finding the best mortgage and home equity loan rates.
njactb.org for finding taxes, ownership, and sale price history on property
eppraisal.com for getting an idea on a property value
rentometer.com for finding out about the rent on specific property... more
Beyond building a client base, social networking allows for efficiency, ease and the exchange of motivation, thoughts and goals - all the tools that make a business work. Though I have been a huge fan