FACEPILE

There's a few different modes of the Facepile plugin. The best thing I can recommend is reading their full documentation on the page below:
developers.facebook.com/docs/plugins/facepile/
Specifically:

My plugin isn't displaying anything
The plugin only displays friends photos. If the person viewing the plugin does not have friends who have liked your page or have connected to your site, no faces will be shown.

If you are displaying people who have liked your page, and nobody has liked it yet, the plugin will render nothing. If some people have liked your page, but none of the viewing person's friends have liked it, the plugin will display the total number of people who have liked your page.

If you are displaying people who have connected to your site, and none of the viewing person's friends have connected yet, the plugin will render nothing.

Hopefully that explains when things will, and won't show. In general, the user at least needs to be logged into Facebook. They shouldn't have to authenticate on your site though.. but will only see avatars of *their friends* who have authenticated on your site or Like your FB Page (depending on what you've configured it to show).

Alex