Check if a class exists within a method

I’m new to OOP and I’m writing my first plugin. I want to check if the Facebook plugin has already been activated. If so, I want to skip some code:

class MyClass {
...
    function fb_js_sdk_setup() {
        // Check if Facebook plugin is activated 
        if ( class_exists( 'Facebook_WP' ) )
            return;
        // Continue if Facebook plugin is not active
        ...
    }

}

My if (class_exists()) statement isn’t working. Appreciate any advice and pointers. Thanks!