Cloak is something of an anti-Foursquare. You connect it to other applications (including Instagram and Foursquare itself) and then get a map of any of your contacts who are near you, along with an alert when they are within a particular radius.
There are some major limitations. As yet it doesn’t support Facebook, which seems to be the most likely outlet for people you choose to befriend online (or feel obliged to do so) but wouldn’t always want to see in person.
More to the point, the technology is neutral and the “avoid people” angle is really just a matter of perspective. There’s no reason you couldn’t pitch the same app as being a way to find which of your friends are nearby, which is pretty much FourSquare.
Co-creater Chris Baker does have form on antisocial technology. He previously created a browser extension that removes pictures of babies from your Facebook newsfeed.