I think they use Perforce (or at least, a heavily patched version they can "Source Depot"). That might have changed since Team Foundation was brought out. I don't think they ever used visual sourcesafe.
I'm in Office and we currently use Source Depot, which is a modified version of Perforce. We have so many tools that interoperate with it (and everyone knows how it works) that I don't see us moving away from SD anytime soon.
We use Team Foundation for a few other things related to project management (personally, I'm not a fan), but not for source control.
We use Team Foundation for a few other things related to project management (personally, I'm not a fan), but not for source control.