With the introduction of Facebook Chat I have found myself in discussions with people that I do not currently have on any of my IM clients and haven't chatted to in years other than very asynchronous messages now and then. Due to the large community of Facebook I think this has the potential to therefore really be useful to many.
Facebook Chat currently relies on a user having Facebook up and this being the current focus, but as soon as:
a) the web client is unleashed from this constraint,
b) Facebook provides an alternative desktop client and/or message alert, or better yet
c) Facebook opens up the API for allthen I think we will see a decline in IM client use or IM client vendors rapidly trying to provide a frontend for Facebook.
With Microsoft's investment in Facebook, perhaps there is already work going on behind the scenes in this space already.