Is facebook going to completely remove xid attribute, replacing it with href only? I hope not, because xid is a lot more flexible than href. Is facebook going to drop all <fb:... > tags? Or are these tags going to work after 1 Jan 2012, but we will call it XFBML, not FBML? Is an official statement somewhere available?
I am getting pretty desperate after spending many hours trying to sort this out.
It looks like the answer to your question is yes. From the link:
We are deprecating FBML. On Jan 1, 2012: FBML will no longer be supported on Platform. June 1, 2012: FBML apps will no longer work. All FBML endpoints are removed. If you are building a new application on Facebook.com, please implement your application using HTML, JavaScript and CSS.
I'm not a Facebook developer, so it's hard to say what you would migrate to in the JavaScript SDK/Social Plugin realm.