The recently announced partnership between Zillow.com, a Seattle-based real estate Web site, and Yahoo! Real Estate has created the largest real estate network on the Web, according to comScore Media Metrix, an Internet audience measurement service.

Zillow now sells local advertising across both sites, and the 4 million for-sale listings on Zillow also appear on Yahoo! Real Estate.

Real estate agents and brokers can buy local advertisements on both sites with one call to Zillow, which has nearly 16 million monthly unique users. Yahoo! Real Estate is one of the largest individual real estate sites on the Web.

“We’re thrilled to be providing agents and brokers with outstanding new opportunities to extend their reach on the largest online real estate network,” said Spencer Rascoff, Zillow CEO.

Rascoff continued, “One phone call can now connect industry professionals to the biggest real estate network on the Web. The launch of this partnership will simplify advertising decisions and bring Zillow listings to millions of additional users every month.”

In addition to selling local ads, Zillow also sells national display advertising across both sites to new home builders, real estate agents, and brokers, and Zillow is now Yahoo! Real Estate’s exclusive provider of for-sale listings. Any for-sale listing that appears on Zillow will automatically appear on Yahoo! Real Estate.

The two sites’ relationship began in 2006 when Yahoo! Real Estate incorporated Zillow’s Zestimate home valuations for more than 72 million U.S. homes into its user experience.

“This partnership brings together the scale of Yahoo!‘s audience with the depth of Zillow’s real estate listings,” said Greg Hintz, head of Yahoo! Listings. “No other real estate Web sites are better suited to bring such a vast offering to agents, brokers, and advertisers.”