As ezl pointed out, the topic is too big for this format, so...the short version.
Trulia started as a scraper site to pull info that is NOT in an mls. That is why they first opened in NYC where there was no mls and at present they pull foreclosure data which is also not mls listed property. They do other things, but they were not trying to merely replicate data already available via "the mls" (multiple listing service owned and operated by brokerages collectively by private membership)
Zillow made their big splash with the Zestimate and then branched out after they had lots of eyes on the site via the zestimate before they moved into for sale property data.
Both are what we call "bottom feeder" sites. The big player is Redfin.com and I don't see you mentioning them. They started as a more data driven tech experience before they farmed out or bought into a lot of their tech work. That is the model you want to look at. Redfin takes mls data plus 3rd party info regarding schools...walk score...many other startup and generally bottom feeder site data and incorporates it into one display...including the zillow zestimate link. They did not intend to be a brokerage initially and shifted gears as the money was in the consumer's pocket and not the brokers' and so they followed the money and shifted to an RE commission model to capture the commission dollars vs being paid by brokerages.
As mentioned here they primarily tried to use tech to reduce the cost of service. They have had to increase their commission from their original idea once or twice and elevate the agent component as well. Their original idea was to make real estate a totally online experience. They are the model to study as to what didn't work and to emulate as to what does.
There is lots of room for more Redfin.com models. There are also many small companies here in Seattle trying other ideas...but Redfin is the one you should all study before continuing with this discussion. People first used them for the tech and not the commission discount. I don't even bother with a website as I only talk to my clients in Redfin links, even though I could easily bring them into the mls private portal.
Their original concept was to take 2% market share. A modest goal. They have far exceeded that goal and have grown BUT they do not go to cheap markets generally where the commission discount makes no sense business-wise for the company vs the consumer. Lower commission only works in places where home prices are high or at least above the national median.
One of my clients sent this link to me and asked for my thoughts. Please excuse the intrusion.