What are the biggest challenges of designing/scaling for a social network with hundreds of millions of users?
The biggest challenge here is that you have no specific audience. Your audience is everyone. For mobile specifically, we have different types of people using different apps on different platforms on different sized devices. Wanting anyone in any situation to have the best possible experience is a really tricky problem to solve.
Without an audience, or trying to make the best thing for every single person who can click on a keyboard, maybe this is why facebook’s aesthetics are so god awful.