I doubt it's based on userbase expansion so much as it is a bet on massive monetization.
Personally, with their existing model, I think that's tough. Unlike search, the monetization rates just won't be that high, since it's not a point where a user is trying to solve a problem they're willing to pay for.
But they might surprise us, and if they find a good monetization model, that's a lot of collective person-hours of exposure.
I think you've hit the key - if they find a good monetization model. I remember an article on here about Facebook shutting down a social shopping experiment, but if that had panned out it could have been huge. There are always rumblings of "social search" every once in a while. Likewise, it isn't hard to imagine communications changing in the next 10 years to a system where your wireless carrier is just a data connection and Facebook could potentially power VoIP and video communications over that.
But I think the valuation is quite high for a potential that they haven't shown they can leverage their users toward. Google hasn't shown that it can leverage its might toward, say, social. However, Google's valuation isn't so high compared to its earnings. For me, it's a big bet to assume a company can drastically change its monetization.
So it's based on the assumption that to date, they've failed to efficiently monetize their user base? They've had advertising for years. They're not going to start charging users. Shopping stuff is great but it's basically affiliate sales.
Saying "we're super popular, and we're going to figure out the monetization later" is pretty bubbly. It's equivalent to "ignore the financials."
Personally, with their existing model, I think that's tough. Unlike search, the monetization rates just won't be that high, since it's not a point where a user is trying to solve a problem they're willing to pay for.
But they might surprise us, and if they find a good monetization model, that's a lot of collective person-hours of exposure.