I think this 11.1 million figure is more reliable.  I looked up the history of the page and the 6 million figure was provided by a user who was not logged in  there's just an IP address.  Furthermore, the 6 million figure is supposedly only the number of users in the US.  I think the 11.1 million number is actually more revealing of the Facebook's growing worldwide popularity.  I'll add it to the article.