It also downscales all the media down, making it hard to appreciate a lot of things.
The main facebook site makes a lot of media requests to preload the things you're going to start reading immediately.
the main facebook site doesn't attempt to scale to the connection speed, which is unfortunate, but it offers a much better experience for those who have the bandwidth to pull in the pictures. Especially for the primary use case (scroll a bunch to see a bunch of people's stuff)
I wouldn't want Facebook to scale things to my bandwidth. That gives me less control over what I see because the system is making mostly arbitrary decisions for me.