Now that Facebook's IPO is well into the past, and user-growth has stalled a touch, it's easy to sit back and take the incredible story of Facebook for granted.

Don't.

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.

First, there's former Facebooker Andy Johns, who writes this:

Read Quote of Andy Johns's answer to Facebook Growth and Traction: What are some decisions taken by the "Growth team" at Facebook that helped Facebook reach 500 million users? on Quora

Next, there's Chamath Palihapitiya, Johns's former boss, who writes:

Read Quote of Chamath Palihapitiya's answer to Facebook Growth and Traction: What are some decisions taken by the "Growth team" at Facebook that helped Facebook reach 500 million users? on Quora

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