Just asked somebody at Facebook who works on this stuff - apparently they do care and it is a balance between a giant freaking download or increasing HTTP reque

Just asked somebody at Facebook who works on this stuff - apparently they do care and it is a balance between a giant freaking download or increasing HTTP requests. There is also the issue of redownloading a giant file whenever a small piece of JS changes on the site, which can happen quite happen. They've also profiled the browser performance itself and found that loading all their JS in-memory is actually a performance issue. So I guess the answer is that they've really thought this through and came up with the best solution that works for them.