Zillow.com has pretty much grown on its own since Rich Barton and Lloyd Frink founded the company in 2005, preferring to build rather than buy. But that’s changing today as the Seattle online real estate company announces the purchase of Postlets, an online service that allows agents, property managers and other real estate professionals to distribute listings across 13 real estate sites. It marks the first acquisition in Zillow’s history.

Postlets topped 500,000 users earlier this year, and it already had a deal to syndicate listings through Zillow.

In a blog post, Zillow CEO Spencer Rascoff wrote that agents have flocked to Postlets because it is easy-to-use and free.

“Agents love Postlets because you create a property listing just once, and Postlets automatically distributes it across the Web,” Rascoff says. “Plus, when you create an account on Postlets, you get a personalized landing page so you can showcase all of your “Postlets” in one place. You can then customize your personal gallery with a company logo and an “about me” page where you can describe your services and upload a personal photo to help you gain even more exposure.”

Zillow’s Rascoff said that Postlets will continue to send listings to its distribution partners as part of today’s deal. Those include some of Zillow’s primary rivals, such as Trulia. It will be interesting to see how Trulia responds to the buy given the intense competition between the two companies.

One possible outcome is that Trulia goes after vFlyer, which competes with Postlets. (The way Trulia and Zillow go back and forth, that wouldn’t surprise me).

Backed with $87 million in venture capital, Zillow.com now attracts more than 19 million unique visitors per month. (19.4 million for the month of March to be exact).

Postlets was founded in the same year as Zillow by Asher Matsuda and Raymond Chen, who started the company as a side project.

A spokeswoman for Zillow declined to comment when asked if we might see more acquisitions by the company in the future.

UPDATE: Brian Boero over 1000 Watt Consulting offers some great insights on the deal, writing that it “underscores just how intense the battle to acquire, control, monetize and measure listing data truly is.” And he too wonders how Trulia will respond:

“If I’m Trulia, right now I’m either pissed, happy that Zillow felt compelled to do something it surely did not want to do, or looking for my own pipeline to control.

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