Facebook itself says it has “More than 500 million active users” on their Press Room page.
According to the United States Census data of 2010, there are 309 million people in the United States.
That means there are more people on Facebook than in the United States.
Although a surprising fact, it is not inconceivable, since 70% of Facebook’s users are outside of the United States.
To corroborate those numbers, lets check some other sources…
According to stat found on onlineschools.org, there are 500,000,000 active Facebook users as of 2011.
U.S. World Population Clocks says population of United States is 310,920,549. And population of the world is 6,903,350,805 (as of 5:37 UTC March 03, 2011).
Doing some math, it means about 7% of the world population is an active user of Facebook.
Hence, it is fairly certain that Facebook has more active users than the number of people in the United States.