There is already a way to buy a prospect's attention and it's to give a $50-100 gift card or Amazon gift certificate. Facebook doesn't take a cut. At lower ends there are small iTunes gift cards, Starbucks and so on. Qualifying prospects is difficult because of the preference to approach without paying, and because people who estimate their time loss worth less than the gift will sometimes lie about their suitability in order to receive it. Because of all these considerations, it's always going to be a narrow market, and Facebook needs to focus on huge markets to make enough for their investors.
My point is that the $1 message would only want to be seen by people who would make more from it than they value the time spent to read it, and companies would only want to send to people who they want to reach for the money, and could not reach for less - and the region between those two constraints is shrunk by the middleman's cut. I'd guess that mostly the only people who want to sit through $1 spam are on a boring conference call or unemployed, people who aren't good prospects. The better prospects would rather do an hour of consulting or work on a promotion.
