[eluser]BrianDHall[/eluser]
Take a look at OpenID, and I think over in Ignited Code (or somewhere in this forum) someone created a library for CI to make working with OpenID easier.
Depending on your subscription to such a service, it controls what data you can access about a given user. The free version gives you the basics, and as you go up in subscription price you could end up with a level of exposed data that 10 years ago would have been considered a horribly unacceptable invasion of people's privacy - but now is just so normal no one really cares
