Nicholas Carlson writes:

Facebook, which now has a billion users, reached 100 million users in just 10 quarters. 

It reached 500 million just 12 quarters later.

That's beyond mind-boggling.

How'd it happen?

Over on question and answer site, former Facebook employees are telling stories about it. They're pretty fascinating – and certainly full of lessons for anyone else trying to build an audience/userbase.

Here are the full answers by Andy Johns and Chamath P on Quora:

http://www.quora.com/Facebook-Growth-and-Traction/What-are-some-decisions-taken-by-the-Growth-team-at-Facebook-that-helped-Facebook-reach-500-million-users