Zillow and Trulia are considered to be the pacesetters in the domain of housing and real estate. Though they are not into selling homes but they have brought in a lot of value to the domain by making the process of buying and selling homes a lot more transparent and insightful with their online advertising services.
The idea has been best explained by Jonathan J Miller in the Bloomberg opinion section where he describes how apps like Zillow and Trulia are changing the way real estate works. This elucidation is well supported by a joint study by the NAR (National Association of Realtors) and Google, depicting the fact that 90% of home buyers do an online research when looking for real estate.