Once in a while, Facebook mobile gets fouled up on my smartphone and stops working correctly. I've had it happen both when I had an iPhone and now that I have an Android LG G3.
The symptoms vary, but usually the problem involves repeated crashing when trying to do a specific task. Today, my symptom was that I was unable to post a comment on an existing post. The app would either crash, or it would fail to post and switch back to the home screen.
The Facebook app stores a database on your phone that keeps track of all of your settings and also caches information from pages and profiles you've visited to speed up access. Once in a while, this database becomes corrupt, and that is usually to cause of odd behavior. Sometimes this is triggered after an update to the app, but other times is just happens from use. Regardless, it can be very frustrating.
Most often, the solution is to delete/remove the app from your phone and reinstall it. When you delete a smartphone app, the operating system knows to delete all of the associated files that go with the app. When you reinstall it, the app should create new, clean database and preference files. However, in some cases, residual files are left behind, which can prevent the reinstall from helping to solve the problem.