When attempting to login to the Facebook app you get the following error "An error occured during the sign in. Please try again later."
It seems that this is most common for new Facebook accounts or for those (like me) who recently changed their Facebook email address to their Gmail address. There is an extensive thread on one of the Motorola boards.
The work around for me was to Login to Facebook from a computer and add my old email address (or a new one) back to the account. Then go to the Facebook app in Settings, Applications, Manage Applications and Clear Data. Then relaunch the Facebook app, agree to the terms and login using the email you just added. Back in!