I've been getting something similar to this when using Mobile Facebook. Commenting on some posts is damn tricky because there's no return button to start a new line and the text doesn't auto format as you type and if you notice you've made a typo in the middle of the text you can't go back to correct it. Everything works fine on the built in Facebook for Android, but trying to use FB's mobile page via a browser has the problem.
Interestingly enough the very same issue raises it's ugly head when I use FB's mobile page on the Nokia N900 too, although because the N900 has a hardware keyboard going back to correct typos is possible..