Facebook has 1.11 billion active monthly users including 751 million mobile users

"Here’s a full rundown for Q1 year-over-year user growth:
- Daily active users (DAUs) were 665 million on average for March 2013, an increase of 26% year-over-year.
- Monthly active users (MAUs) were 1.11 billion as of March 31, 2013, an increase of 23% year-over-year.
- Mobile MAUs were 751 million as of March 31, 2013, an increase of 54% year-over-year."

Source:  Facebook Q1 2013 Earnings reported by Techcrunch, 1st May 2013