You cannot use new in the declaration of attributes. So you will have to set it when you need it. As you might want to use it from static context, the constructor might not be appropriate (it only gets called when you use new on your class and you will have different instances of Fb in each object).
It could be better like this:
public static function getUser() {
if (self::$user == null) self::$user = new Fb();
return self::$user;
}
And then always retrieve the static attribute via
self::getUser()
