On a site that I'm working on, I'm trying to find ways of enforcing a one-person, one-account rule. In general, we'd like to do this by providing options to authenticate users with third-party services that provide this assurance. For example, it's possible to authenticate with Facebook and check whether the user is considered "verified" by Facebook (which means they must have provided either a phone number or credit card). This is roughly the level of identity verification we require--we're not doing banking or anything like that. But we want to give the user options. My question is, who else, besides Facebook, provides this?

(uncertain of the proper SE forum, please comment if there's a better SE site to ask this)