The problem of 'scale' they were addressing in the iOS app wasn't scale of users, but rather scale of devs. They have 18k classes because they came up with a rather clever way of allowing many separate developers to all work on different features of the same app, without stepping on each others toes. This might not seem like a hard problem if you've never worked on a non-trivial iOS app, but it is a pretty impressive feat.

Facebook wants the shortest possible time between product idea and real world users interacting with it, which is why they also have a solid CI/CD pipeline. You might not agree with the tradeoff, but the pragmatism is impressive