The Facebook sizes are 720, 960 or 2048 pixels on the long edge. Those are the sizes that FB advise for best quality.
Nobody uses 720 any more, it's just so small.
2048 is lovely and big, if people are viewing on big screens, but overkill of course for people viewing on phones, which is most people now. Plus 2048 is printable, so it's dangerous from a piracy point of view - you need to make sure you've watermarked carefully. Do you add a watermark?
960 is pretty good. But of course watermarking is a good idea all the time anyway.
The 11:15 shape is only for images which you're selling. For Facebook, you don't need to restrict yourself that way - you can crop to any shape that suits your photo best, you know?