I read a post "post comments using twitter and Facebook" where these this trail is supposed to succeed: blog’s admin → Settings → Discussion. Near the bottom of the page you’ll see a “Comment Form: Prompt” setting. (at http://en.blog.wordpress.com/2011/06/07/post-comments-twitter-facebook/)
I see "comment form" and "prompt" on my discussion page and there is a form field "leave reply" next to it but there are no options to select and nothing about Facebook or twitter.
Blog url: http://absolutelyblabulous.wordpress.com/