If Facebook were to just disappear tomorrow all of humanity would be better off. I am not saying it would solve all our problems but I think it gives such a boost to humanity in so many spheres that it instantly pushes us on a much more positive trajectory.
If we're talking about general good, I cannot for the life of me imagine why anyone would pick Facebook as "the thing" to disappear. I mean aside from the idealistic candidates like Wars or Guns, what about cigarettes, alcohol or all the other drugs that are widely used, have clear connections to depression and most of which will also literally kill you if overused? I really doubt getting rid of Facebook would do the world more good than any of those candidates. But of cause freedom of choice and let each pick their own poisen etc.