Is there any good sample about facebook-php-sdk3.0+ with CodeIgniter2.0+?
Check the name of your controller. If it is called Facebook then that is your problem, you cannot have 2 classes called the same thing at once, unless they are in different namespaces.
I wouldn't suggest using the normal Facebook PHP-SDK when there are a number available over on codeigniter sparks that perform most of the same functions but already setup to work with codeigniter without these conflicts.