Yahoo says it will begin to roll out Facebook Connect across all of its properties in the first half of the year—and says that will “create a loop that drives visitors back to Yahoo” when users share Yahoo content on the social network. The deal comes as Yahoo CEO Carol Bartz has talked about making her company’s properties more “social.” And, indeed, AllThingsD’s Kara Swisher says it’s possible that Yahoo could roll out similar integrations with Twitter, LinkedIn, and MySpace.

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Facebook, meanwhile, gets to establish Facebook Connect as the web’s top identity system (Om Malik has more about that in his analysis here). And, indeed, the deal is a nod to Facebook’s growth—and its dominance of the social networking landscape. While Yahoo still tops Facebook in terms of unique users by a substantial margin—both in the U.S. and worldwide, according to comScore—the social network continues to grow at a heady pace. Just last evening, Facebook announced that it had added 50 million users over the last two months-and-a-half.

As for Yahoo’s own social efforts, they have faltered. The company has shut down both Yahoo 360 and Yahoo Mash in the last year-and-a-half, although it talked earlier this summer about possibly making an acquisition that would let it become a larger player in the social networking space.