I think what will end up happening eventually is that the price of using Facebook will be 1 message delivered to you each day from a targeted company.
Advertisers will bid against each other for that coveted message each day, and Facebook will figure out how to maximize bids. For example, even celebrities will be targeted, so companies may end up paying $10,000 per message for them, while a typical person may be $1-$5, or something like that.