You may be grossly overestimating ad prices. A datum is damn near worthless, but data in the aggregate is valuable because of the scale of these companies.
User tracking seems to be far more valuable than ads. I'm currently subcontracting for a specific large media trust. Ad contracts are in total about 6 figure numbers. User tracking, sponsored content and skins (custom stylesheets with a sponsor's branding and tracking) are very substantial 7 or 8 digit contracts.
While it's true his own activity isn't worth much, it's still a piece of a very lucrative pie. There are services already oriented specifically around user tracking.
I guess they're oing by the principle of individual behavior is unpredictable, but mass behavior is.
