Not 12 billion - the number just needs to greater than the number of computer/internet users on earth.

If it's close (8 billion), people will likely ignore it as either a rounding/math/counting error, or as "those people who have a 2nd account for their dog".

There are 7 billion people on earth, there are 2.3 billion with Internet access as of 2011 - FB claims that 40-ish% of Internet users are monthly actives, which is clearly BS, so you don't need to get it anywhere near 8 billion. You simply need to get it near or above actual Internet access capable people.

So more like 4 billion.