Lol. No. Hardly anyone uses facebook chat. It doesn't support Video (Yahoo!, WLM and Google Talk does, on Mobile and [WLM only on] Desktop) or Voice and it also doesn't support much as far as File Transfer is concerned. The fact that FB has a ton of users doesn't mean more people use their chat functionality than Yahoo!.
Additionally, Yahoo! and WLM are interoperable, so when you use one of the two, your potential friendlist is the size of the two combined. In addition to that, WLM and Yahoo! both integrate with Facebook Chat so you get that as a bonus as well. Google Talk interops with AOL Instant Messenger, IIRC.
I doubt more people use FBC for IM than Yahoo, WLM, or Google Talk. I really do. Sure, more people do use Facebook itself, though. Their IM apps are absolutely terrible.