Kelly from Trulia here. This is an important question as we measure our success by the value we deliver to you. Not to make this a blatant sales plug, but since you asked...here are just a few data points:
1) Volume - Trulia was a top 3 referrer to 12 of our largest broker partner Web sites, according to Hitwise data from this Fall: http://www.truliablog.com/?p=207
2) Quality - The Trulia user is a highly attractive audience of serious home searchers - take a look at my blog post with results from our recent Trulia user survey: http://www.truliablog.com/?p=216
3) Measure by Others' successes - there is a great case study no Prudential Fox & Roach reporting that Trulia users were clicking to their site and requesting double the average # of showings than other user: http://images.trulia.com/resources/Trulia_Prudential_Fox_And
And if you purchase a monthly subscription to Agent Featured Listings you get: contact info displayed, ability for clients to email you direct, listings placed in the top 3 search results and weekly reporting you can forward to your clients. Plus, agents receive 4x the normal Trulia traffic on average through this product! http://www.trulia.com/agents/feature
Hope that helps.