ART is where android is going, but the turnover is by no means official. Some apps aren't entirely compatible. My Facebook works fine. Try a reinstall, or clear its cache. It should work. The TW app may not be up to par yet, not too surprising. Again, maybe a reinstall or cache clearing.

 

There is an ongoing discussion of art here if you haven't seen it yet

http://androidforums.com/showthread.php?p=6251042

I'll just mention its likely going to be an important part of the android experience in the future, but its not ready for primetime yet. I wouldn't suggest your average person use it, which is why Google buried it in developer settings