There are already plenty of apps which do similar things, like foursquare, but they tend to focus on locations rather than people (x went to this restaurant, etc.)

As you describe it, I don't see why I as a user would want other users to know who i've been hanging out with. If you can find a way to do it that doesn't seem quite so neo-Orwellian then there might be some value to it, otherwise other apps are already doing it better and I think you'll have quite a lot of network effect to overcome.