It's one of those ideas that seems obvious... after someone else thinks of it. Public health researchers in New York City searched Yelp for complaints that sounded like food poisoning. Only 3 percent (15 of 500) of the suspicious incidents had been reported to the city. The researchers caution that it took a lot of work to review and follow up on the Yelp complaints, that they weren't able to identify any of the infectious agents in the outbreaks they found, and that despite all their work they still may have missed cases.
So this experience indicates that while social media may be useful in identifying food borne illness outbreaks, it's not simple.
Read the full report in this week's MMWR from the CDC: