Facebookが独自の認証システムFacebook ConnectをやめてOAuthを採用する。
Facebook Adopts Open Standard for User Logins | Webmonkeyツ†| Wired.com
そのために単純で簡単なOAuth 2.0を採用する。

“Developers aren’t asking for OpenID,” Shepard said when the question was posed to the panel. “They’re explicitly asking for us to make logins simpler and easier, not for us to implement OpenID. So now we’re doing that by implementing OAuth 2.0, because it’s simple and easy. Adding OpenID on top of it would just add a layer of complexity nobody is asking for.”
OpenID is indeed very complex, and because of that, it suffers from usability problems that have kept it from being widely adopted.
“It’s very easy to do user authentication over OAuth 2.0,” Shepard said.