We use Twitter to promote our site. We also bring a substantial number of people here to Trulia.
Pinterest, Tumblr and AboutMe share information about us.
I use main FB & Twitter along with a few chosen blog communities and that works well for me.
You really need to be on every single social media site there is...in the end, they all create 'juice' by driving traffic to your site, your listings, your blogs. The higher you rank, the more inquiries you get. Real Estate is a contact sport, so you need to be out there constantly JUICING your brand.