Just accept that a DDoS might happen and that there's nothing you can do about it. It's fine, it's just how the Internet works.
Most time I get ddosed now it's either Facebook directly, Something something Azure or any random AI.
And lots of real users time wasted for captchas.
Does it make sense? Nah, but is it part of the weird reality we live in. Looks like it
I have no way of contacting Facebook. All I can do is keep complaining on hackernews whenever the topic arrises.
Edit:// Oh and I see the same with Azure, however there I have no list of IPs to verify it's official just because it looks like it.