itsallkizza is correct. Ajax also includes iframe requests however, I know what you meant.
Facebook uses iframe and XMLHTTPRequests all over the place and all the time. If you install 'firebug' for firefox and click the console tab you can see all the requests flying.
I can only think they use XMLHTTPRequests for things that are 'nice have' items but not required for functionality, where they use iframes. For example, XMLHTTPRequests is used for chat, notifications and inbox updates, on page status updated.