I figured out why Facebook chat won't work in m-im.
Facebook chat requires DIGEST-MD5 authentication that is not supported by m-im. (Implementation would have been easier if Java ME included a MD5 library...)
I logged an issue asking for support if anyone is interested. (My patch only adds support for detecting whether the server supports DIGEST-MD5 authentication)
I like m-im, mostly since it is based on MGtalk, but has a better user interface, and because it supports multiple accounts (although not at the same time, which would have been ideal).