It’s easy once you’ve seen it done.
Open the iPhone Facebook application.
Select the button at the bottom that shows the user’s profile. (Should be the second button from the left!)
On that page at the very top in the left-hand corner there is a button which says Logout.
Click the logout button 🙂
You’ll be asked to confirm, and then you will be told you have successfully logged out of Facebook for iPhone 🙂