FB can grow either by increasing it user base as you suggest, or by monetizing it's users better. So if it say increases usage by 5x and monetization by 5x you are looking at a 25x multiplier. Will this happen, I have no idea.
Unfortunately for Facebook, this is fighting a few trends. users numbers have gone down in the US and Britain [1] and mobile usage is going up [2], and Facebook is having a much harder time monetizing mobile over the web.
I agree with your general premise however, that Facebook's primary growth won't be on the user base, but needs to be on the revenue per subscriber numbers. This is going to really test one of the Valley's most recent premises, that building a massive user base will also create a massive business (Twitter, Instagram I'm looking at you...)
