When saving WP will perform a certificate check (to get app access token). If you have not upgraded the certificate lists on your PHP install, this will cause that error.
You can either update the certification authority lists in your PHP install, or you can add the ‘sslverify’=>false flag to the arguments list for wp_remote_get to make it ignore the certification verification.
Goto line 263 in ../wp-content/plugins/facebook/includes/facebook-php-sdk/class-facebook-wp.php
...
$response = self::handle_response( wp_remote_get( self::$DOMAIN_MAP['graph'] . 'oauth/access_token?' . http_build_query( array( 'client_id' => $app_id, 'client_secret' => $app_secret, 'grant_type' => 'client_credentials' ), '', '&' ), array(
'redirection' => 0,
'httpversion' => '1.1',
'timeout' => 5,
'headers' => array( 'Connection' => 'close' ),
'user-agent' => self::generate_user_agent(),
'sslverify'=>false,
) ) );
...
NOTE: You also need to correct at the line 92
