Define stable. Do you mean it crashes? or that it drops out once in a while? Because I use Facebook over XMPP and I've never actually experienced any problems.
It dropped a lot when I tried using it, and the "in browser and Adium and a Pidgin" combination was super unstable -- caused it to crash a lot more. It may have improved, but I couldn't get it to stay connected more than 15 minutes. This was about 2 years ago maybe?