@Dagobert If you sign up to any website using Google+, Twitter, Facebook, so on, all associated accounts are cross-referenced. Furthermore it cross-referenced to your private contacts list, as in Reddit could see Gmail contacts if you used Google login, and can even see your phone contacts if that Gmail account is the same one connected to your phone. This sounds like what may have happened.
Everyone should always do a regular email sign-up and avoid doing the lazy one-click sign-up, because it will turn a small ban on a single site, to a ban on all sites associated with the email.