I registered my account with facebook on askubuntu, but now I'm deactivating my facebook account. What to do to stay registered on askubuntu and change my login details?
Add a new login before you nuke your Facebook account.
If skipped that and now you're stuck without a login, I'm sure the nice people at Stack Exchange might be able to help you provided the other account information (email, etc) is valid and really is you. Use the contact us link at the bottom of the page.