Yahoo Inc will let users of its email, photo-sharing and other online products link their content and activities directly into Facebook, the world’s No. 1 Internet social network. Yahoo’s support of the Facebook Connect service, which it said it expects to begin in the first half of 2010, represents an important move in Yahoo’s efforts to tap the popularity of social networking and underscores the growing clout of Facebook, which now counts more than 350 million worldwide users. Earlier this year, Yahoo allowed users to preview messages from their Facebook friends directly on the Yahoo homepage, as part of a broad revamping of the Yahoo front page. Wednesday’s announcement takes the integration between Yahoo and Facebook a step further, by automatically pushing activities performed on Yahoo sites, such as photos shared on Yahoo’s Flickr, into the Facebook news feed. Yahoo said it expects that the content that consumers share on Facebook will drive visitors back to Yahoo.