Thanks for taking time to reply to comments here.

If I understand correctly, when facebook detects faces, it does something like...

    compare face to all Face Templates
    if match
        associate picture and face with matched user
        (maybe also update Face Template of matched user)
    else
        discard data structures created during compare
        (maybe mark face as "already compared")

If the data is truly discarded, great! I'm glad if my earlier speculation is not what happens.