Enabling "Posting as" option with Facebook Comments plugin to post as Facebook Fan Page?

How do you enable the Facebook Comments plugin to display the "Posting as" option, which allows you to post comments as a Fan Page instead of your personal profile?

We use the same comments plugin code on different pages, but for some reason, the "Posting as" option only appears on one of the pages. It makes no sense why it appears on one page and not other pages.开发者_如何学编程

Any clues?

Finally found the solution: if you leave the "href" attribute empty, you lose the "Posting as" option. This is for the Javascript/XFBML version.