by default the composer autoload is set to
FALSE. So open
/application/config/config.php and replace this:
$config['composer_autoload'] = FALSE;
$config['composer_autoload'] = FCPATH . 'vendor/autoload.php';
FCPATH is defined in the root index.php file, if the vendor directory is at the same level, then it should work fine, otherwise define the absolute path to the autoload file.
Now, I have not tested the Facebook SDK with CI3, so I'm not sure this will solve your issue. If problem persist then show us the error codes.