With more than 350 million users, Facebook is winning the social networking race by a landslide. But another area they’re winning big (in large part due to that growth) is in portable identity, where their Facebook Connect platform competes with offerings from Google, Twitter and MySpace.

Today, Facebook is getting a massive new partner for Connect: Yahoo. The #2 search engine, which is also one of the world’s most visited Web sites, announced that it will be rolling out Facebook Connect on numerous Yahoo properties that in the aggregate represent tens of millions of users worldwide.

Yahoo says that Connect will be integrated on its Mail, News, Sports, and Finance sites, as well as Flickr, allowing users to do things like leave comments, share news stories, and view friends’ feeds by using their Facebook account on Yahoo. Like any Connect-powered service, Yahoo benefits by that activity being shared back on the Facebook news feed, which in turn drives traffic to Yahoo.

This is a no-brainer for Yahoo, who has been trying for years to make its services more social on its own. Now, they’ll start to see some of the benefits that smaller publishers have seen from Facebook Connect (more comments, extra traffic from Facebook, etc.) on a massive scale. The new features are expected to launch next year.