If you are not already working with Trulia, but are working with Realtor.com, you may want to consider expanding to Trulia as well. They are growing at warp speed right now in terms of overall traffic. They are quickly outpacing Realtor.com and Zillow in terms of audience growth...see below.
The chart above illustrates a 156% audience growth rate for Trulia year-over-year compared to only about 20% for both Zillow and Realtor.com for the same time period. Their month-over-month growth rate is double digits which is amazing considering they are driving about 2 million people to the site every month and their competitors are posting 1.5% and 2.2% growth rates.
I expect their growth to cool somewhat as they mature, however they are already posting great traffic levels at these growth rates. It may be quite a while before any significant slowdown occurs.