Online real estate can be divided into 5 types of businesses. First are sites that allow people to search the MLS (multiple listing service). Second is the field of property valuations. The third offers demographics and census mash-ups. Fourth are community based forums and news services. Fifth is the world of FSBO (for sale by owner), which is divided between sites that offer a FREE FSBO service, and the others which do not. If you are trying to succeed in online real estate, work with the top sites in any of the 5 verticals.