Re: So hows this "hack" work exactly ?
If a scammer were to run this as an actual operation, they'd probably use some phone API like Twilio, which will dramatically reduce their costs. Then they would just request a new number, attempt to log into Facebook, and if the login is successful, take it over, rinse and repeat.
While the "come back! text" tipped me off that this was possible, it's not necessary to know if an account exists that that given phone number.
You are correct that there is an element of luck involved. But I stumbled across enough accounts in a row that I'm wondering if I'm either incredibly lucky, or there's a lot of accounts up for grabs.