During its video calling announcement today (you can watch the whole thing here), Facebook confirmed that it has over 750 million users. Facebook co-founder and CEO Mark Zuckerberg said that the social network didn't report the milestone because the companies doesn't believe the metric properly gauges user engagement. He did, however, mention that he hoped that Facebook would reach a billion users in the near future.
Last month, it was first rumored that Facebook had 750 million monthly active users. At the time, Facebook stated that it didn't have anything to announce in regard to its user base.
On January 5, 2011, a Goldman Sachs report to potential investors leaked out, and quoted 600 million users, but this was unofficial. On July 21, 2010, Facebook CEO Mark Zuckeberg announced the site had passed the 500 million user mark.
Ever since then, Facebook has been saying it has over 500 million users. Now the company is officially referring to its user base as over 750 million users.
The next official number will likely be 1 billion users. After that milestone, the next one will be passing India, and then China. If Facebook was a country, it would be the world's third largest, by population.
Plenty of websites only report cumulative users, which does not take into account inactive accounts. Facebook's user count metric is much better as it is defined as a user who has logged in within the last 30 days. Still, this does not eliminate the issue of one person having multiple accounts.