You can also go to the Contacts app, click on Groups in the upper left corner, scroll down to "All Facebook" and uncheck it. This gets rid of people who are only Facebook friends and weren't already in your contacts list, but it does leave photos and Facebook email addresses that were added to your existing contacts.
I don't see Groups in my Contacts app. The upper left of my Contacts app has an arrow in a circle, but tapping on it does nothing.
I also went into my iPhone's settings under Mail, Contacts and Calendar, but couldn't find a Groups button there either.
How do you find Groups in your Contacts?