I didn't reactivate my account but I got an email from Facebook saying that I did.
If you received a Facebook reactivation email, it's possible that someone attempted to log into your account using your email and password. If this happens, we recommend that you reset your password to help secure your account.
You should also make sure that any email addresses you use with your Facebook account are secure. Remember that anyone who can read your email can also potentially access your Facebook account too.