I think its a context switch for the user. When a user goes onto Quora they are looking for information. Then, when they see the FB connect button, that goal

I think its a context switch for the user. When a user goes onto Quora they are looking for information. Then, when they see the FB connect button, that goal of finding info suddenly becomes related to social relationships. Now the user has to switch mental contexts and think about the social impact of their decision. That invokes an emotional response when all the user wanted to do was quickly find an answer to a question (or at least explore a site that might have this answer). I've done user tests with FB connect buttons. I've seen users actually pull their hand off the mouse and take pause when the button appears. It invokes a powerful emotional response in some users.