Use Reports or do a Query directly on the TMG logs could basically display what user is visiting a specific website, and to some extent, you could see what they are posting i belive, depending on how the POST is done(ie if it's included in the URL),
but i don't think you would achieve what you're looking for.
As Anders said, it's quite a mess on the legal front to inspect users traffic this closely without a reasonable cause.
Basically, TMG doesn't log all data that is sent, only information about the traffic source, destination, size, method and so on.
Ben wrote a post about the logs:
http://blogs.technet.com/b/ben/archive/2012/01/06/querying-the-tmg-log-directly.aspx
So a web filter or a second Proxy would probably be the only technical way to get that information. But the logs would be HUGE unless there is a feature to actually only log the data that is relevant (NSA style). =)
HTH
Patric