The main issue isn't the gmail or what not has been breached. It's that lots of users tend to reuse passwords. So...once they know you signed up using foo@gma

The main issue isn't the gmail or what not has been breached. It's that lots of users tend to reuse passwords. So...once they know you signed up using foo@gmail.com to service Alpha (and they have that password)..then they start trying all of the common services to see where else foo@gmail.com might have used that password or a slight variant (dropbox, etc.)