Ever wondered how Zillow/Trulia works?
Its a simple website aggregator that gets 95% of its data from the local MLS (Multiple Listing Service Listings) and rest from direct postings. They sell overpriced ads that most agents do buy because they simply dont work and have crappy ROI. They basically sell ads to the same agents tho provide them these listings. Go figure, a middle man for another middle man. Then when you throw Google in mix you got another middle man.
Its entirety depends on MLS to feed the property data and Google to be found online by consumers looking for these properties.
If MLS or Google ever wanted to pull the plug on the big Z will be left eating a lot of shit crumbs. These two companies have Zillow on a lifeline.