A large percentage of companies don't do their post-market surveillance, and the FDA has never even once revoked approval because of it. The whole FDA approval process is basically a scam.
Are you confusing it with post-marketing commitments to run additional clinical trials? That's not the same thing as post-marketing surveillance.
[1]http://www.fiercepharma.com/story/roche-overlooks-80000-adve...
"Under the current system, the FDA has the authority to revoke approval of some drugs for which Phase IV commitments have not been honored. This authority has never been invoked."