Facebook drew 133.6 million unique visitors in January, while Yahoo pulled in 132 million visitors. Google still leads the way with 147.8 million visitors, or nearly half the U.S. population.

Facebook is king of the Web when it comes to the amount of time people actually spend online, however.

"In January, 11.6% of all time spent online was spent on Facebook (compared to 4.25% for Yahoo and 4.1% for Google)," Compete says.

Twitter is also on the upswing, according to separate research released this week.

Twitter attracted 21.79 million unique visitors in January, an increase of 9% over the previous month, according to an article in Venture Beat.  The traffic increase has mainly been driven by Google, which recently started posting Twitter status updates in search results.