Not to detract from your main point, which is very sound, but on this: > whereas that walk out by the lake with Sarah If you both have the Facebook app with l

Not to detract from your main point, which is very sound, but on this: > whereas that walk out by the lake with Sarah If you both have the Facebook app with location tracking on, Facebook does know about this, which is why you sometimes get friend suggestions for people after you spent time near them (eg you were at the same party or they visited your house to do cleaning or plumbing), even if you have no mutual friends.