There, I did it. Haha.
Can we stop beating dead horses, we all read Hacker News around here?
For now, the best you can hope for is a layered defense and rigorous dev and ops practices to help minimize the attack surface and reduce the overall damage a single successful attack can achieve.
Automated testing/fuzzing could find this, but probably better training/practices would be easier to get right and save time/money in the long run.
While Facebook most likely does do some form of threat modeling for their main site, without a rigid process for all code that goes public you'll run into issues like this that are just as severe. Just because it's a mobile support site for requesting photo removals doesn't mean it is less important surface area in terms of security.
Testing can only ever go so far - bugs and vulnerabilities exist everywhere, even in Facebook.
Regardless, well done!