If you are getting an error every time you try to login to the Facebook app on your iPhone, try this solution.
- Go to http://www.facebook.com/iphone
- On the left-hand side (you may need to scroll down) click "Remove app"
- Now try logging in again on your iPhone.
I found the solution here: http://goo.gl/LPYEu
So what caused the problem?..
Not exactly sure, but this started happening right after I update my Facebook security setting. I had turned on HTTPS and had activated alerts for when a new computer or mobile device logged in to my account. One of these settings must have caused it.