I am in the business of buying and selling, so I use Big Crumbs for social marketing. I need shoppers, and the site is geared toward bringing in shoppers, and getting discounts for them. It's great free advertising for my store.
They also have forums and a meet people and make friends feature. You can post bulletins to them to promote other programs:
http://my.bigcrumbs.com/FunStuffbyMinny