No bias here as I don't have any affiliation with Zillow/Trulia/Redfin but...

I feel like the only real disruptor in this market has been Redfin - the only one with actual real estate agents.

Zillow and Trulia are, put extremely simply, a map with ads on it. Useful tool, absolutely, but not going to shake up the real estate industry, which is undoubtedly a dinosaur.

When you look at a company like Redfin, who is an actual brokerage but with non commissioned agents, and lower commissions across the board, they're actually disrupting a market that has been ripe for change for years, and they now are offering a better service/product at a cheaper price. I'd expect big things out of them, and expect several brokerages will either change gears to match them or more competition to pop up here soon.