Yes, I think that's basically it. Right now the valuation of companies with little or no revenue is based on future cash flow from users. That makes the calculation of how much one user is worth much harder since 90% of users will just drop off and use the next free service.

Also, it's really hard to predict how much users are willing to pay for distraction and entertainment. There are some sites I get value from (e.g. buying/selling or booking things) but the vast majority is just entertainment.