Re: can be saved by two factor authentication
"google/facebook/whoever will now get your phone number every time your friend clicks the "sure I'd love to give you full access to my address book!" button"
Data mining's a bitch aint it?
Frustratingly there are now plenty of apps - LinkedIn, Strava, FitBit, ... which ask to scan your contacts list to see if you already have 'friends' on the service they can link you to - or more likely they can lure onto the service on the basis that you have joined.
Many of these services incorporate Facebook and Gplus authentication, which means that even if you created your account directly with them as I do, your details have likely been shared with these mega corporations. ("known as 'carefully selected third parties' in the terms and conditions)