After much trial, and nearly as much error, I managed to get the 1.65 version of pidgin-facebookchat installed on Ubuntu Hardy Heron. At this point, it appears to be working smoothly without any other impact on the operating system, though your results may very, so be careful. In other words, don’t hold me responsible if your system crashes, eh?
You actually need two .debs, in the following order:
Install them using the package manager of your choice (I just double-click to bring up gdebi), restart pidgin, and you’re all set!