A month later, you're Googling for a local STI clinic and have no reason to believe that clicking on the top result (LocalTimes' eclassified section) is going to share your interest in local STI clinics with all your social network. But it is, because LocalTimes' single signon site which you're already logged into makes no distinction about what is and isn't to be shared, but they have made efforts to adopt this revolutionary new form of passive sharing which is going to push up their pageviews.
On another note, I'm seriously considering closing my FB account once and for good and leaving for G+. I have a good crowd of friends willing to do this (and invite their friends), but G+ offers no way to import FB data, which would be a nice feature.
G+ makes as little sense for a private person as Facebook, if not even less.
It's not as though could be browsing one day in search of "abortions Chicago" or "genital warts" and have that auto-published to your FB page. You'd have to give an app explicit permission to share activities on your behalf.