Think: modeling a marketing scheme against FB's data because they understand the idiosyncratic specifics of the demographic you are trying to hit, thus telling you with p<.01 whether or not it will be profitable.
That, and government contracts are profitable.
Also, 'viable' would mean different things in different contexts. I think it is certainly viable for the current investors to exit with a good return on their investment if the company goes public.
For the company itself, it can be a different story and we need more information to comment on that.
Advertisers = Customers
Their valuation is based on potential, not just the current revenues.