Not very often, let's say 20% of the time, I decide that the signing in some site to comment on a blog or create a quasi-account using Facebook is unlikely to b

Not very often, let's say 20% of the time, I decide that the signing in some site to comment on a blog or create a quasi-account using Facebook is unlikely to be a privacy problem: I'm willing to give site X my main email address, even to receive newsletters and commercial offers. At this point, Facebook asks me to share my friends list (often much more) with site X: the only rational response is scrambling for the "close tab" button in the browser and writing off site X as untrustworthy jerks, because they dared to ask for unneeded personal information.