If anything, Credit Karma ARPU could be higher. Credit card referrals typically range from $50 to $200 dollars. Massive amounts of users come to Credit Karma through Google, sign up for a credit score/card, and then never come back.
This valuation would imply they refer at a rate of ~1-5 million cards a year I guess.
Investors care less about actual profits than the ability to prove profits can be generated. Once it's demonstrated/proven that profits can be attained, investors become extremely patient. As long as potential profits keep rising, the enterprise value will rise, and investors will be happy. This is not the Warren Buffett approach, obviously, but that doesn't make it necessarily wrong or invalid. It's consistent within the theory of rational expectations.
They'd have a $15-$20 billion valuation today if their revenue were that high in the private stage.
LendingClub is growing incredibly fast, is tracking toward $350 million - $400 million in annual revenue, and is valued at $6 billion. The public market is harsher on valuations than the private market as well. Were LendingClub still private, its valuation would probably be twice what its public valuation is.
Credit Karma is more likely in the $50m to $75m range on revenue.