Is it permissible to guess the abandoned Facebook accounts by recovering the email after it becomes available, and if that is forbidden, how can these accounts be returned to their unknown original owners?

It is not permissible to hack a Facebook account, even if it was created with an inactive email, because Facebook has kept the account, and its owner has the right to retain it. If the questioner is unable to return the account to its owner, repentance and freezing or abandoning the account are sufficient for him.