Well, I ran a couple of tests to see what this was all about. I have a FB account but have never found a use for it, so I was curious about the photo sharing capabilities.
First I selected a 3648x2736 RAW file and directed Lightroom to upload it to FB with the following settings: JPEG, 80% quality, no resizing, sharpen for screen. After the upload I went to FB to view the resulting image. It looked OK in the FB viewer, so I selected Download. What came down was a 2048x1536 pixel image of 767 kB,
Next I selected the same RAW file and directed Lightroom to upload to FB with these settings: JPEG, 80% quality, resize to 2000 pixels maximum, sharpen for screen. Repeating the view and download process, what came back was a 2000x1500 pixel image of 810 kB.
As a final test I had Lightroom export the same image to my hard drive with the same settings (2000x1500 size). The result was a 1.24 MB file.
All three downloaded images look essentially the same on screen.
Not sure what all this means but at first inspection it would appear that:
1. Maximum image size on FB is 2048 pixels.
2. FB does apply some additional compression to images.
3. The 720 pixel limit is contraindicated.
(In case somebody does the math and wonders what is going on, the images are 4:3 aspect ratio because they are from a G11.)