[madeofpalk]

I don't think such a seriously scary thing like this would remain a secret for long. Not only would I think that someone inside of Facebook would leak it (just like the Trending News debacle that Gawker covered), but I strongly believe that security researchers or other people fiddling around with things on their phone or the Facebook app would uncover it.

Combine that with permission models on most devices that alert the user when the microphone is turned on, I cannot see Facebook doing this secretly.

Also, just think about all the things that your girlfriend said around her phone that hasn't turned up in Facebook ads. I'm willing to chalk this up to a weird coincidence or other reasons mentioned here.