I use facebook to arrange outings, simply because it's easy - most of my friends have it and use it regularly - and its a good place for me to store and talk about photos, which I can access from anywhere with unrestricted internet access (lol school internet censorship), although that can possibly backlash severely.
Another thing I find quite useful is the private message system, in which you can send a message to a selected number of people, and the message acts like a private forum. It's immensely useful for group assignments, as it has one main advantage of chat rooms - the conversation is stored and can be accessed at anytime.
The idea of paying facebook is just ridiculous to high school students such as myself, simply because we don't have any way of paying online without an adult, and dear god I would never ask my parents. So with facebook gone, I lose a pretty useful organisation tool.