The difference between why the iPhone Facebook app can use chat and why BB's facebook app can't is because (I believe) the iphone app was developed by Facebook. The BB app was developed by RIM.
Facebook obviously has access to their own chat API. They have not released a public chat API that 3rd party developers can use therefore no chat is available on the BB version.
Unless Facebook opens up the API or RIM specially negotiates with FB, you won't see chat on the BB app.
also wanted to throw in there that you seem to love IM if you have multiple IM screen names you use on different messengers something like this might be good for you as well.