Looking for Facebook/Twitter comment plugin

I am helping my niece build a website for her annual Christmas charity. We want to use a comment plugin that allows Facebook, Twitter, and WordPress users to be able to comment by logging in. I have not been able to find any suitable plugins at all. If anybody knows any, please do let me know. FYI, I am using a self-hosted version. I am also a PHP developer, so I could do what is necessary to code part of the app myself.