I'm been working on some pages and have been annoyed by the lack of Intellisense and tag completion on the <fb> tags. I searched here and couldn't find any information on making it work. After some searching through the assembly, I figured
out the line:
<%@ Register Assembly="Facebook.Web" Namespace="Facebook.Web.FbmlControls" TagPrefix="fb" %>
Just add this to any aspx page that uses the <fb> tags and you'll have auto-complete and no compile-time warnings from unknown tags.