What can we do now that Facebook iPad app crashes on launch after update?
Sadly, unless you have a computer with that accounts signed version of the old Facebook app, you can't get the old version from the iTunes Store. 

Deleting the app should have removed all preferences, so you can only make sure the crashes are being reported to Apple and hope the app is popular enough to get a bug fix rapidly. 

To restore the old app from a computer, be sure to delete the faulty app before connecting the iPad to the computer. Then add the Facebook app from the computer being careful to not update it there.