I'm a few of those active users.
Because let's be fair: Facebook is one of the very few companies that announces active users, with a fairly clear definition of what they mean by "active". At this point they're likely to have more accounts (including unused ones) than there are people alive so it would be silly now, but they've virtually always used the active metric.
They could hit 1.5 billion tomorrow if a group of hackers got ambitious. The number is practically meaningless. Unique individual numbers are far lower.
What would be a better metric in your mind than MAU? Contrast facebooks MAU to google pluses "has a google account" metric.
The facebook prospectus also has daily active users of which at last count there were 483mil.
Worked for a social gaming company, investigated this exact problem ("How many of our DAUs are multiple accounts?"). For our game, which had mechanics that encouraged player-to-player commerce, the number of these fake accounts was huge: around 40% on DAUs of ~150k. It was clear to me and my co-workers that this was a big issue, everywhere. But everyone in the space benefits from saying "1 BILLION USERS!" when you are selling ad reach. Except for those actually buying ads. Oops.
Moved on to different things, shorted ZNGA in April after the OMGPOP acquisition, having been privy to this scheme. Have made a decent return, covering at ~$5, but missed the last two big drops. I'm not particularly concerned what you choose to believe, because my view has worked out pretty well for me.