Need an advice about comments

I am searching a solution for wordpress comments. I like standart comments, I already styled it and made some additionaly functionality. But there is one that I can't find and don't know how to make it. I need an ability for unregistered users to left comments using some social service, . So, I don't want to offer them ability to log-in using their social service, but only to comment.

UPD: I didn't know about such rule. So ok, how can I realize this functionality? I mean authorization with accounts from any social service only to post a comment?