This seems to be based on the count of tags being resolved.
For example when viewing the "featured" question for the tag filter *facebook*
, this is resolved to
http://stackoverflow.com/questions/tagged/facebook+or+facebook-graph-api+or+facebook+or+facebook-like+or+facebook-c%23-sdk+or+facebook-javascript-sdk+or+facebook-fql+or+facebook-php-sdk+or+facebook-opengraph+or+facebook-apps+or+facebook-social-plugins+or+facebook-page+or+facebook-comments+or+facebook-iframe+or+facebook-authentication+or+facebook-oauth+or+facebook-canvas+or+facebook-fanpage+or+facebook-wall+or+facebook-login?sort=featured
and should list three, but shows only two "featured" entries: