[rejoinder]

I got freaked out by Facebook because I signed up there using a work email.

I think Facebook's recommendation system was using the data from other users' address books. As in I'd get a list of various people that I'd done business with in the past as suggested friends on Facebook. Those people had my address in their address book and I can only assume they'd uploaded their contacts to Facebook. Facebook retains that data. Here I am a new user. Wow how does it know I know such and such? I can only assume that it also works the other way: I join and I pop up in their account.

Google got into a mess with Buzz and the address book leaks, and email address leaks.

So there's a fine line here between privacy and helpfulness.