There's nothing stopping you from using an anonymous Twitter account to login if the site supports Twitter authentication. Alternatively, MyOpenID and some oth

There's nothing stopping you from using an anonymous Twitter account to login if the site supports Twitter authentication. Alternatively, MyOpenID and some other OpenID providers allow you to choose what pieces of data to present to the requesting site when asking you for permission to authenticate with them. Facebook is another story, as they want every profile to correspond to a real-life human and only present an allow/deny choice for permissions to the requesting site instead of finer-grained access control.