For the most part, Social Network sites are "after market"-style tools.  That is to say, they are great for keeping you in touch with your client base in order to update them on your goods and services, but they rarely generate brand new leads.  In 10 years and many, many sites, I've had 2 people contract me based on such sites, and one was an abstractor looking for work.

  Perhaps I am marketing it wrong, as that is always a possibility.  I've set up real estate and researcher groups of shared interests on some site and joined existing groups on others.  I've upgraded services on a few in order to have a bigger presence, and advertised on others.  They seem to be time-sinks, and generators of get-rich-quick emails to my main box, and little else.