This past December we reported on how Facebook was coming up on Blogger to steal its top spot among social media sites when measured by total unique visitors worldwide.

Now, it appears as though Facebook has finally done it. Data from comScore, which unfortunately goes only through December 2008, shows how Facebook’s visitors (221 million) basically matched Blogger’s (225.5 million) by the end of the year. That’s a gap of just 4.5 million versus the gap of 21 million that existed in November. Assuming Facebook’s upward trend continued in January (and Blogger’s remained flat), the social network sits on top of the roost now.

Facebook stands out from the others not only because it’s the most popular; it’s also the only one showing consistently healthy growth. Chris DeWolf in a recent interview with Charlie Rose said how he doesn’t expect Facebook to pass MySpace by 2010 in terms of U.S. unique visitors (as we’ve predicted in the past). But that prospect is still inching towards reality, as shown by the graph below.