Good point. I hope this doesn't become yet another forensic pseudoscience tool were procedures that are very much prone to error, are treated as infallible simple because they are use technology which the common man doesn't understands.
Context. Their preferences, friends they hang out with, brands they identify with, writing style, etc. will probably be different. These are things that Facebook can see.
That's far into the realm of sci-fi for now, given that basic face recognition is still quite poor. See my other comment for details: http://news.ycombinator.com/item?id=4275921
It probably won't. However, with the numbers of cameras that are are being deployed everywhere, soon you'll only have to "tag" them once, and then you'll be able to track them as they move around the world.