My sample-size-of-a-few take is that it's a combination of factors:

1) The probability that a Facebook users at somepoint has had a bad experience with rogue Facebook app publishing personal information (or even an accidental acceptance) or was witness to a friend's experience is non-trivial

2) The increasing awareness of the scope and scale of personal data that is taken from logins and shared with advertisers

3) The demographic shift in Facebook usage to older users

4) The large secular increase in awareness of ongoing privacy and financial information breaches that rightly or wrongly is associated with over-sharing of personal information