>> Facebook could easily send such messages only to people who support one particular party or candidate, and that doing so could easily flip a close election – with no one knowing that this has occurred. And because advertisements, like search rankings, are ephemeral, manipulating an election in this way would leave no paper trail.

I would be interesting if someone setup some kind of monitoring that might detect if that happens. They'd probably need the cooperation of many legitimate and diverse Facebook users, though.