I had a 5-letter username that hackers would constantly try to take. They somewhat succeeded this summer by having my account deactivated. There is no real recourse on my end since Instagram/Facebook doesn't have customer support.
My real fear is that someone will try to impersonate me in the future.
I think you were looking for a support email. I think at facebook's scale its a bit difficult. I would recommend you to head over here https://www.facebook.com/help/1306725409382822 and someone will help you out. This page can be easily found in our Help and Support section
Only because facebook is greedy as fuck and prefers massive amount of unethical profits than actually providing a good service.
That's probably because you're not the customer.