Efficacy.
I know I've told this story before, but I'm boring that way.
Like every other male on Facebook, I kept getting adverts for "Hot young promiscuous women in YOUR AREA inexplicably want to meet you!" Kept rejecting the adverts as offensive (which, since Facebook knows I'm married, they are), to no effect. Then one day my wife got our daughter the DVD of Annie, and I happened to make some comment on FB about a silly detail I noticed in the film. Immediately, all those ads vanished, to be replaced with "Lithe young men in stripy leggings and leather caps in YOUR AREA want to meet you!" Facebook are supposedly one of the world leaders at personal data-mining, and they have an algorithm that goes something like IF [mentions a musical] THEN [gay].
They have also served me adverts for a motorcycle hearse -- for all those Hells Angels' funerals I'm always organising -- and (and I swear I am not making this up) an amphibious assault vehicle. I'd love to claim I'm exciting enough for these adverts to have been pointed at me for a good reason, but I do IT for banks and like cooking and gardening.
To be fair, I did also discover the music of Meiko via a Facebook ad, and she's now one of my favourite singers. But I'm guessing that's just because her fanbase is mainly gay bikers mounting amphibious invasions.
I do object to the invasion of privacy in principle, but I'm not worrying too much about its actual effects just yet.