I don't know if you're joking or not but that is exactly true. Meta went as far as creating their own PHP engine and then a new PHO compatible language because they didn't have the resources to switch from PHP.
Instagram is presumably in the same position. Switching language is basically impossible once you have a certain amount of code. I'm sure they were aware of the performance issues with Python but they probably said "we'll worry about it later" when they were a small startup and now it's too late.
Well, Facebook also created their own hacked up version of PHP (called Hack) that's presumably easier to migrate PHP to.
Hack is actually surprisingly pleasant, basically about the best language they could have made starting from PHP. (I know, that's damning with faint praise. But I actually mean this unironically. It has TypeScript vibes.)
I was excited about Hack when it came out. Unfortunately PHP took just enough from it to kill it. I gave up on it once Composer stopped supporting it, after backwards compatibility with PHP no longer became a goal.
IMHO Hack's best feature was native support for XHP... which (also unfortunately) isn't something PHP decided to take.
I only used Hack when I was very briefly working for Facebook. (And I used PHP once before nearly 20 years ago by now for some web site I had 'inherited', back when PHP was truly an awful language)
Instagram is presumably in the same position. Switching language is basically impossible once you have a certain amount of code. I'm sure they were aware of the performance issues with Python but they probably said "we'll worry about it later" when they were a small startup and now it's too late.