There are echoes here of the "Beacon" fiasco --- in which Facebook was briefly broadcasting users' activity on third-party sites (particularly on-line purchases

There are echoes here of the "Beacon" fiasco --- in which Facebook was briefly broadcasting users' activity on third-party sites (particularly on-line purchases) to their Facebook friends, whether the users wanted that broadcast or not. This isn't quite the same thing, but you can certainly imagine it going wrong in some of the same ways...