...well, you do need to restore your contacts first after a flash. Then, when you log into Facebook you'd just relink your contacts.

So...restore your data and contacts. Then go to the People Tab, click "All People" and go to the Events [social networking] Tab. You should be able to sign in and relink everyone.

If that login fails, try logging into Facebook through your contact card. Once you're logged in, you'd only need to go to that Events page and it automatically compares your facebook friends with your stored contacts...that's how they get linked.

If you've done this before, you'd do the same thing after a flash. As far as I know, there's no way off a fresh flash for the device to automatically sign you into Facebook and update itself. ...and even then, you'd need contacts on your phone first for Facebook to link up.