Facebook has more than 750m monthly active users

"Mark Zuckerberg confirmed that Facebook had surpassed 750 million monthly active users today at a product launch in Palo Alto.
He said the company had declined to announce it earlier because it’s become focused on other metrics, including how actively users are sharing information.
“Hopefully, we’ll get to a billion at some point,” he said. “I think people generally think that’s going to happen at some point.”
The announcement suggest that Facebook’s growth, at least in terms of raw monthly actives, is continuing at a linear, not exponential, pace. The company has been growing at just under 50 million monthly actives every two months since late 2009. It last said it was at 500 million monthly actives a year ago."
Source:  Inside Facebook, 6th July 2011