If you want to see ALL the homes that meet your search criteria, you're much better off searching on www.DaveSuttonHomes.com (which uses the Windermere search engine) than either Zillow or Trulia, and probably any of the other real estate "syndicator" sites that accumulate listings from a wide variety of sources.
In Portland, only 80% of the homes for sale appeared on Trulia, 81% on Zillow. So using one of those sites to search for a home missed one of every five homes listed. The Windermere site had 100%. Why the difference? The study showed that the median time between a home being listed and appearing on Zillow was seven days! Nine for Trulia! On www.DaveSuttonHomes.com it's 15 MINUTES or less!
In Portland, median days on market for the most desirable homes is now not much more than 12. Recently I've held two homes open in NE Portland. For each I prepared a list of homes sold in the last 90 days within a quarter mile of that home. In both cases the median days on market for the over 30 homes on the list was 13! Half of those 30 homes sold in 13 days or less! So a lot of the homes are sold before they ever appear on Trulia or Zillow.
That's just one more good reason why you're better off to list your home for sale with Windermere, too.
The reverse is true, too. Over one in three homes shown for sale on Zillow (36%) or Trulia (37%) at NOT actually for sale. Windermere is not perfect here. 1.7% of homes on my web site were not for sale either.
I'm not here to bad mouth Trulia and Zillow – they do have their place. I've answered over 600 questions on Trulia and will continue and with Zillow as well, but this information comes from a study by the WAV Group, one of the largest real estate consulting firms in the nation, titled The Accuracy of Real Estate Websites
The whole research report is available here. It's actually pretty readable as research reports go.