It's not a matter of simple price per user. At its core, their product is the incredibly precise and nuanced network they have constructed though their various

It's not a matter of simple price per user. At its core, their product is the incredibly precise and nuanced network they have constructed though their various sources of user data. The value they get per click/view is only so valuable because companies are willing to pay for the quality of targeted advertising that only a company on the scale of facebook can provide. This model allows FB to keep their platform free for users, lowering the barrier to entry meaning that more people are likely to join the network, whose data makes FB's advertising system all the more effective. More data means a better user experience (and better advertising), which naturally draws in more users to add their data to the pool. The end result is a virtuous cycle that provides companies with this model the best data, the most users, and the best targeted advertising in their respective fields. This model is the endgame of companies like Facebook, Gooogle and Amazon, and is the reason FB isn't interested in your $1.