As Facebook announced on the 10th of February this year, they are depreciating the use of FBML starting tomorrow. This means that all of your created welcome landing pages you are yet to deploy in the FBML-application will not be possible from tomorrow (11th of March) onwards.
Existing FBML-boxes do still work
We do not know for how long, but the existing FBML-boxes will work tomorrow as well, but you do good in keeping an eye on the development roadmap at a regular basis to see announcements on when FBML will be completely removed. You might also see this as a great time to start talking to your clients about that you need to rebuild as Facebook has made this update.
What’s the alternative to FBML-welcome pages
In the same post they write about the new types of Facebook iframes that they will be using from now on. It is actually really much better than the old version as you have a lot more freedom now to create webbapps with a wider range of functionality.
I will try to get better at posting tutorials again and my first will be an update to the iframe post I wrote a while back.