Facebook not working on my iPhone

You will have to ask the developers of Facebook about that. it is not an Apple product. Contact Facebook --> https://www.facebook.com/help/

If instead you meant Facetime: If you can’t turn on or sign in to iMessage or FaceTime, read this support article --> If you can’t turn on or sign in to iMessage or FaceTime - Apple Support

Also try restarting your device. That cleared up an activation issue we had, and it doesn't hurt to try.

If it still isn't working after a day, do as the article instructs and contact Apple --> Contact - Official Apple Support

Facebook seems to be having issues with their iOS/iPad apps the last few months. I suggest that you access FB through your browser instead, at least until FB can get their act together.