Totally insane idea. It’s fine to spy on other people so long as they don’t spy on you? Here’s an idea, don’t sign in to the location service in the first place. If by some incredibly unlikely chance someone you don’t like buttonholes you at a cafe, tell them you don’t like them and don’t want to talk to them.
This skulking approach to life is not only paranoid and soul-destroying but morally speaking it is pretty much stalking in reverse. Moreover, for those few cases where it might be justified – harassing exes and so on – the data quality of foursquare and services like that is very low, so it wouldn’t be safe to rely on it anyway.
