I have the same problem
I am on fw 2.1
and I've deleted facebook app by accident and now when I try to download it again I get the error message :this app is not compatible with your iPhone.
I cannot upgrade to fw 2.2 (I bought the iPhone jailbreak and the seller told me to never upgrade or may "break" the iPhone ("Please remember DO NOT update firmware through iTunes on your iPhone as the device may lock up and become unusable. And all 3rd party software (video recorder) and all the Games will be deleted. You have been warned ")
So what can I do?
Is there a way to download the facebook app for version 2.1 ?
Is your phone unlocked or just jailbroken?
What the seller told you is not true. What he meant was if you do the updates in iTunes, you will erase the JB. This is fine, you just need to rejailbreak each time you want the new FW updates. There is no way around this. It's safest to restore to factory settings rather than just install the updates. Starting fresh each time prevents traces of the JB from staying on the phone which can potentially cause problems.
If your phone isn't unlocked, restore to factory settings in iTunes with the 2.2.1 FW and then rejailbreak. If the phone is unlocked, restore to factory settings with the 2.2 FW and then rejailbreak. You can get 2.2 here if you need it. http://www.iclarified.com/entry/index.php?enid=750
It's unlocked since you bought it in the US and aren't using AT&T.
You can restore it to factory settings with the 2.2 ISPW, rejailbreak it with QuickPWN, and then unlock it with yellowsn0w. Do NOT upgrade to 2.2.1 or you won't be able to unlock it and the phone features won't work. Be sure to use the 2.2 ISPW. Download and save it to your desktop before restoring. I put a link for the download in my post above.