As a real estate agent, you need to concentrate on building two types of networks.

The first should be real estate related.  Concentrate mostly on larger well trafficked sites that are either hubs or authorities.

The second should be geographically related to your service area.  Geographically, be as "targeted" as you possibly can - not a wide region - but the local community where you do most of your business.  Make sure the web site mentions the local community in the web site title and the page content.