I've personally had this play out with a number of services. LastFM comes to mind.

I really enjoyed having a persistent record of what I listened to for my own use and for finding new music.

However, I stopped using the service because, honestly, I'm not exactly proud of every song that makes it to my most-played list. LastFM let's you remove a song from your profile if I remember correctly, but it was easier to not share at all than to remove specific types of sharing.

Facebook has this problem times 100. I try and tell everyone I know that isn't technical that Facebook knows every website they've been on that has a like button on it (and then I make a serious face and say evverry site) most people have no idea. The idea that Facebook has this kind of info and is going to start "autosharing" makes me uncomfortable.