I read on Wikipedia that it uses libpurple, but I can't figure out how to connect to other services (like jabber for example). Am I missing something or is the feature just not there anymore?
Clients of libpurple don't have to use all the protocols, or have them as options. Looking at the QuteCom wiki and various documents I could find, it looks like they're using libpurple for specific backend/frontend handling and their client doesn't support other protocols.
Have you tried connecting to QuteCom's backend using Empathy?