Maybe I'd buy this if you used Google+ as an example instead. As far as I'm concerned I get many times more spam on Facebook than through email - only once every few months does a message from a Nigerian prince slip through gmail's spam filter, but multiple times a week I get a FB message that someone I vaguely knew 3 years ago lost their phone and needs numbers, is having a party thousands of miles away, etc.
Facebook encourages its users to add lots and lots of friends to your main friend pool, and always has. It's spent most of its existence trying to get lots of users and build a huge social graph. Having hundreds upon hundreds of "friends" is the norm, not the exception - there's no emphasis on keeping your set of facebook friends small and tight-nit.
