I think Facebook do know what they're doing, but are perhaps putting energy into the wrong things. Using a ton of JavaScript to avoid ever having to load another page is a nice idea, but it ends up bogging down the client. Using a lot of abstractions away from the DOM makes things slow. An occasional page load is probably beneficial.

I'm also unconvinced they can't scale down images server-side to make them more mobile-palatable.

because they're a big company with a lot of staff, everything they do must therefore be "the right way"? That's a fallacy if i've ever heard one.