How to call wp plugin REST functions without curl?

You can internally route REST API requests using rest_do_request and a WP_REST_Request object.

$request = new WP_REST_Request( 'GET', '/fb_connect' );
$request->set_param( 'x', true );
// You can also use it this way:
$request['y'] = true;
$request['z'] = true;

$response = rest_do_request( $request );