And this is not something new. Back in the days of ICQ and hotmail, it was trivial to "hack" someone's email account by filling in the required information to "recover your password" by using information users stored in ICQ (which usually included their email).

I remember searching for people with an @hotmail.com account in ICQ just to see if I could enter to see their emails. This was like, 15 years ago IIRC.