Try this -- On your homescreen, tap on Contacts. Press Menu > More > Display Options > Select Contacts to Display

Uncheck all options except for Facebook contacts. Tap Done.
You will now be directed back to your Contacts where now, only your Facebook contacts are displayed. All other contacts saved on your phone or SIM card and contacts synchronized from your email accounts have all been hidden.

Tap on Menu > Delete > Select All > Delete > Ok. All Facebook contacts are now deleted.