I think Zillows market share is a bit bigger but I find them to be a bit of a train wreck, have been for years. Despite that many buyers use the site. I have an issue with inaccuracy and many of these sites are notorious for that. They "scrape" info from various sources that may not be accurate or up to date. I personally get more interest from Trulia and have been active on here since 2007.
I have answered this question before and the answer still remains, Zillow has not generated a single interesting lead for me, ever. Does it have anything to do with location? Maybe. I get leads from Trulia almost on a daily basis, good and not so good, but at least you know you are alive.