I can't tell how it works from the site, but a good approach is to randomize the distance threshold a bit each time. For example: if users are within 1km, always connect; if more than 2km apart, never connect; 1.xkm apart, connect with probability x.

That kind of thing combined with some rate limits (how often one can attempt to connect to a user) should be effective.

(edit) Actually, I think I like mseebach's suggestion even more: For each user, pick a random location about 100m away from them, then pretend they're there until they've moved far enough away, then repeat.