Why doesn't Facebook serve contextual ads against messages?

When our video producer, Richard Blakeley, sent me an invite to the Belmont stakes this weekend, a link appeared below his message. It read:

Gambling Problem? The New York Racing Association encourages responsible wagering. If gambling is a problem for you or someone you care about, help is available 24 hours a day. Call toll-free 1-800-437-1611 or 1-800-522-4700.

For a bit, we both thought we'd discovered a new advertising product for Facebook — contextual ads against messages similar to the kind Google runs against Gmail messages. Turns out we were wrong. The message came from one of the Web pages Blakeley linked to in his message and Facebook was trying to preview it for me as an attachment. Still, it got us thinking: Facebook will only earn revenues around $300 million this year. Why not serve contextual ads against Facebook messages. People plan a lot of nights out over Facebook. Wouldn't restaurants, movie theaters and bars pay to be a part of that conversation? And with all the talent the company is hiring away from Google, couldn't they poach a few engineers from the AdSense team? Check out Blakeley's note and its misleading attachment, below.

Why doesn't Facebook serve contextual ads against messages?