In Fairness, and only my 2 cents and worth no more ---- I just spent 15 minutes doing a few basic tasks on Zillow's site (with a PC, not mobile), and the usability experience on the site feels a bit disjointed, the main scenarios seem poorly defined, and the Ads overwhelm the actual content in a lot of places (need to have a better line between user Value Add vs partner's spamming Ads (3rd party leadgen)). Heck, even the content overwhelmed the content. Although not apples to oranges, both Trulia & Redfin's site experience felt smoother and more aesthetic (again, I only spent a few minutes, but that's also what a first time user does before they move on).
#1. Finding Zillow -- Grade 70 out of 100. I went to Google, and typed Real Estate Seattle. Zillow was pretty far down. It was a little better for "Mortgage Seattle" ... but not much. Bottom line, Zillow needs to increase *NEW* traffic every single day (you can not hook them unless they find you), thus customers who don't know they need Zillow need to easily & magically FIND zillow in the Search Results above the fold *WITHOUT* actually typing the word 'Zillow' into the search bar. And to be clear, 70 isn't a disaster, but they're leaving thousands of potential users for the competition to exploit instead. Maybe I'm wrong and Zillow tends to get new customers word-of-mouth, or from TV & print ads, but from where I sit, organic search seems to be their only avenue to fuel userbase growth (coupled with new interesting offerings for revenue growth), and there is ample room for improvement.
#2. Using Zillow -- Grade 70. I walked a few base usage cases I thought might be important (but I don't know if my cases are representative of tyypical users), such as I would like to find a rental in the U District, and I would like to find a new home in Fremont, and I would like to find a mortgage broker. Zillow starts out great, intuitive, but very quickly it feels overwhelming ...complicated. Made my eyes hurt. The revenue producing userbase requires a high noise & pain threshold to get simple tasks done. I realize we're not a free app, we need to make money, but that doesn't negate my statement. Re-focus on the key use cases, make them simple and gorgeous (half of this Realty game is all about image).
#3. Cross Selling for benefit of partners -- Grade 85. Zillow has got a lot of good hooks, and first and foremost is figuring out the right location, and then making sure Z revenue links are right about where they eyeballs will be. This is why the stock price even exists today, so yes, do more (expand on Mortgages, expand on Agents, ... but the danger might be that all the Zillow R&D investment focuses on this one revenue stream, and when it hiccups, we get clobbered. aka: We become a 1-trick pony that sneeze's a lot.
#4. Aesthetics -- beauty is in the eye of the beholder, and my personal take is anyone who actually might be on Zillow for the sake of commerce has to have a high tollerance for NOISE (as well as above average network bandwidth). A lot of things pop up and get in the way of simple tasks. A classic example is that mammoth 'Filter' box --- over crammed overkill, and it came up like 7 times on me in 15 minutes ... it was #$%$ me off by the 3rd time.
... I'll cut it off there as without a deep dive, most of this is subjective anyhow.
You forgot something: Zillow beats out Trullia in every category you mentioned and the rest of the "competition" is "off the radar anyway. Despite some of the lies you've spread here, Zillows number of unique users continues to clime dramatically.
Have a nice day! :)