Yahoo and Facebook Wednesday announced an expanded partnership allowing their respective users to more easily share activity updates and content with each other.

Through the social network's Facebook Connect program -- which lets members use their Facebook ID to log onto other sites -- Facebook users will be able to share different types of content, such as Flickr photos or comments on news stories, with Facebook friends.

At the same time, Yahoo users will see friends' Facebook activities in "Yahoo Updates," a tab found on key properties within the Web portal such as Yahoo Sports, News and Finance.

The deeper integration of the companies' sites will begin sometime during the first half of 2010, and builds on the current arrangement in which Facebook users can access their "stream," update their status from the Yahoo home page, and share certain types of Yahoo content. It also extends Yahoo's broader strategy, aimed at making the site more open and customizable through alliances with other sites and developers.

"With this integration, we are opening the door for two of the Internet's largest online communities to make it easier for people to stay connected," said Jim Stoneham, vice president of communities for Yahoo, in a statement.

With Facebook announcing late Tuesday that it now has 350 million members worldwide, the two Web giants have a combined user base of some 850 million. That's potentially a lot of status updates going back and forth across networks.

Beyond gaining reach and boosting engagement, how the companies will further benefit over the course of their five-year deal is not clear. Stoneham told The New York Times, "There is lot of potential future integration work we can do." Whether the partnership opens up increased advertising opportunities is an especially intriguing question.

It's already rumored that Facebook plans to build an ad network on top of Facebook Connect. Through such an initiative, Facebook would be able to sell ads across third-party partner sites based on demographic and other anonymous data it collects on users to deliver more targeted ads.

But if Facebook were able to strike such an arrangement with Yahoo alone, it would represent an enormous audience to sell ads against and potentially bring revenue gains to both. Whether Yahoo would be as willing to be as open on the ad side as the content side in that case is another question.

This news story is important. I recommend that everyone read it.