There's no way for anyone to know how long the app support will last. However, it may be possible that services may require an updated app in order to sign in. I can't remember a specific app, but I have had that happen in the past on older iOS devices. I would download the last app version that is compatible with that iOS version. However, whenever I would attempt to sign in, I wasn't able to do so since that specific app version wasn't supported anymore.
However, I expect that those apps will still continue to work in the future. However, any features that FB might add in the future may not work correctly (or at all, or be available) in older versions of the app.