The partnership between real estate websites Zillow and Yahoo! Real Estate is now a reality, if a little late. Last July, the two firms announced that the initiative to have Zillow power all for-sale listings on Yahoo! would be live by the end of 2010. Starting Thursday, any for-sale listing that appears on Zillow, even for-sale-by-owner listings, will automatically appear on Yahoo! Real Estate. At any given time, there are an average of 4 million listings available. "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 websites are better suited to bring such a vast offering to agents, brokers and advertisers." Advertisers can now buy packages to appear on both websites as well. Zillow’s sales force now will also sell local advertising across both sites. Real estate agents and brokers will now have the ability to buy local advertisements on both sites with just one phone call to Zillow. Traffic to Zillow is up. In December, the website reported a record 13 million unique users. In a statement released Thursday, that number is now 16 million. Competition in the space, on the other hand, is waning. Google said last week that it was pulling its real estate listings service next week. Google said lack of interest is the primary reason, though one source who asked not to be identified tells HousingWire that it is routing its resources to fight "the Facebook threat." "Google is more interested in taking Facebook's users, even though these users can't be monetized in the way Google would like," the source said. Thursday's announcement is the end result of five years worth of strategic alliances between Yahoo! and Zillow. In 2006 Yahoo! Real Estate incorporated Zillow’s Zestimate home valuations for more than 72 million U.S. homes. "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. "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." Write to Jacob Gaffney. Follow him on Twitter @JacobGaffney. The author holds no relevant investments.