Just last week, Aol celebrated its re-emergence as an independently-traded company. But its one main advantage is that it still commands a large audience, and in fact was the fourth largest Website in the U.S.—until last month. ComScore data for November, 2009 shows that Facebook surpassed Aol with 102.9 million unique visitors in the U.S., versus 99.7 million for Aol.

Although Aol’s unique visitors grew by nearly a million since October 2009, Facebook grew by more than 5 million. On an annual basis, Facebook’s U.S. audience grew 104 percent since November 2008 (from 50.5 million visitors). During the same period, Aol’s audience declined by 8 percent. That decline is worrisome to Wall Street.

Facebook already passed Aol on a global basis back in February 2009, and became the fourth largest site worldwide last summer when it became more popular than Wikipedia and the other Wikimedia Foundation sites. How long before Facebook catches up to Microsoft’s 136 million U.S. visitors, Yahoo’s 160 million U.S. visitors, or Google’s 168 million U.S. visitors?

Or can Tim Armstrong turn the tide and regain AOL’s standing in the Web firmament?