Is Facebook an OpenID provider?

No, they're not an OpenId provider. They use their own OpenID-like system called Facebook connect, which you can use to authenticate users on your site, among other features.

You can check it out here: https://developers.facebook.com/docs/facebook-login

There is an unofficial Facebook OpenID Provider available at:

http://facebook-openid.appspot.com/

You can eaisly use it to log in any OpenID site with Facebook accounts.

Facebook is not an OpenID provider, they do however allow OpenID login using Google accounts.

OpenID provides a list of the most popular providers.

OpenID is a decentralized authentication protocol. It allows users to be authenticated by co-operating sites using a third party service, eliminating the need for webmasters to provide their own login systems, and allowing users to log into multiple unrelated websites without having to have a separate identity and password for each.

Users create accounts by selecting an OpenID identity provider, and then use those accounts to sign onto any website which accepts OpenID authentication. Here is a list of OpenID providers.

Facebook neither use OpenID nor OAuth as instead of just offering credentials, Facebook wanted to offer friend access and dynamics to the privacy of the information that simply wasn’t possible with the other standards. With Facebook Connect, what we see are elements of both OpenID and OAuth. Facebook Connect can verify that you are who you say you are, and it can then provide access to your data once you’ve given it permission to do so.