Outspoken Zillow CEO Spencer Rascoff made headlines this week by using an industry event to publicly describe his arch-rival, Murdoch-owned Move Inc., as “a crappy company.”

There’s no love lost on the Move side either. Move, which operates the Realtor.com real estate listings site, has previously cut off listing fees to Trulia right after Trulia was acquired by Zillow, and now has Zillow in court over its merger with Trulia.

Certainly, the stakes in the real estate listings data business are huge, so bare-knuckle competition isn’t surprising. What is surprising is that both companies are finding success with radically different business models.

Realtor.com has what I view as a very conventional “just the facts ma’am” user interface. It offers basic parametric search, with listings displayed as summary listings, each offering fast access to listings detail. Real estate agents can pay to advertise themselves or highlight specific listings, and are provided with sales leads as well.

Zillow, as you may recall, burst onto the scene with its “Zestimates,” its estimate of the value of every home in the country. This got Zillow immediate interest and tons of traffic, and it quickly became a major player in the market. Zillow also distinguishes itself with a map-based user interface and somewhat different listing detail than Realtor.com. But the “Zestimates” that helped Zillow rocket to the big time are a two-edged sword. Sellers almost always feel they should be higher, and buyers tend to assume they are much more authoritative than they really are. Zillow also sells advertising to real estate agents with essentially the same suite of offerings as Realtor.com.

Does it make sense that both can thrive? Certainly, we see examples of “co-dominance” in many very large BTC markets simply because they are so large. But while more subtle, it appears that the biggest weakness of both sites – neither has 100% of all listings – may be a strength. That’s because lots of people use both products, leaving real estate agents uncertain about where to place their advertising dollars.

It’s the same situation we saw play out in the heyday of the yellow pages industry. Independent yellow pages directories sprung up everywhere as lower-cost competitors to big, established telephone company directories. But advertisers, rather than cheering and running to advertise in the new, cheaper upstarts, found themselves confused and fearful. Which directory did their customers use? Did they use both? Well, the safest course for many advertisers was to advertise in both directories, meaning their cost to reach the same market went up significantly. Not surprisingly, advertisers were not happy with this outcome.

There are rumblings of discontent in the real estate market as well. Indeed, a new initiative called National Broker Portal Project, meant to be run by and for real estate agents and brokers, is gaining steam. It wants to create a major site that will be both dues-funded and run according to rules developed by the brokers themselves. It’s a long shot to be sure, but it shows once again that being the dominant player in a market is tricky, and sharing that dominance is even trickier. We must all remember that disruption in any industry is not inherently a one-time event.