The
problem there is that '10k+' "engineers" are
trying to make
the same 'photo, video, post and message-sharing website'.
It's a structural problem and little more: the website (and app) is their main money-maker, so they're going to give it a disproportionate amount of resources.
Imagine you hire ten thousand people to lay one railroad track. [note; see end of post] If any single one of them doesn't contribute directly in some way, you'll fire them. This seems kind of strange, doesn't it? Sure, it probably requires more than a single person to lay a track. But ten thousand people to lay one? How is that supposed to work, mechanically? This would be enough to warrant shareholder revolt.
Now, the railroad track gets broken a few hundred times, maybe they hammer it enough to make it twice as long, whatever. It now no longer resembles a railroad track. Certainly no train could go across it. Send a few hundred people to go ask the managers of this project for a replacement track. Okay, we're now at...maybe a tenth of people having contributed? Repeat this process until everyone's contributed. Maybe the manager gives different groups different materials for the track to fuck with them, whatever. But somehow, every single person manages to not get fired.
What's the outcome look like? You have a single railroad track, probably not even well-fit for the job (sparks fly whenever trains run on it; maybe it causes them to tilt, so on), but it's laid! And ten thousand people are employed!
It's the same thing with a website. You can't put a terabyte onto a user's device every single time they load your website; you just can't. So you have a window of performance you have to hit. Between ten thousand people trying to have things thrown onto user devices? Good luck making anything resembling 'decent'.
It's the same problem that Dave Zarzycki noted in his talk about launchd[1], but worse. Instead of 512 megabytes shared between some random abstract parties you can basically ignore, it's <10MB shared between ten thousand coders, translators, graphic designers, users, managers, etc. Does something seem strange about this?
[note]: This is the appropriate comparison here; at the scale of 'Over ten thousand people working on one program', it's grunt work, not art, science, or even programming. There's a word for implementation-grunts that's fallen out of favor in the past few decades: coders. This was seen as distinct until recently.
[1] https://youtu.be/SjrtySM9Dns?t=255