From the Store listing, v3.57.0 just shows "Login fix" as the changelog, but that's a rather important fix, so....!
Updating fMobi in the Nokia Store and launching the app again...
Even if you've authorised fMobi before with Facebook, you'll have to do it again because of the changed permissions system. Don't worry, you won't have to do this again. Well, until the next time Facebook change something major without telling app developers....
Otherwise, the new version remembers where you were the last time you were successfully logged in through fMobi, with all settings intact.
You can buy fMobi for £1.50 here in the Nokia Store or just update your existing installation in the Store client on your phone now.