I think this sounds more like an actual problem with the face book app, as opposed to the phone. Have your tried uninstalling the Facebook app and reinstalling it? That usually fixes problems such as this.
I have had this issue on the S3 and the S5. I wish I had a good solution for you, but the only thing I've found that works consistently is to go to the phone's task manager (the list of items that are active on the phone) and close all. That restarts Facebook and also closes out items competing for memory on the phone, so it gives me better performance overall. I hope this helps!
The Facebook app is constantly being updated and unfortunately not all devices perform as they should. While I've never had a problem with the Facebook app myself on the HTC ONE M9 that I'm using, I know a now a lot of people that do have issues, and a lot of the time they get fixed over time, there's no quick fix solution unfortunately until the next update is made available.