To play devil's advocate, the relevant number is expected future earnings, not current ones: at a 0.95 discount rate, $75 billion amounts to an income stream of $3.75 billion a year. Clearly they're not making that now, but:
1) there's a good chance that they will have far richer data, and therefore the ability to target ads better and more profitably, than google
2) there's a nonzero chance that they will be able to monetize micropayments in a far bigger way than paypal ever did.
There are plenty of other potential revenue streams, but these two alone make the valuation seem like not so much of a stretch — they might just be a $20b company, but then again, they could conceivably end up being a $200b company.
They say they're profitable, but if the profits were impressive, they'd probably publicize them more (obviously not in explicit terms, but they'd describe the scope - i.e "8 digits"). Random guess, $40 million.
