Ahh the meaning of the word useful is "capable of being put to use or serving a purpose, having value or benefit." Facebook is useful in that you can share all different types of info (be it personal, political or business related) with Hundreds of Millions of people. So it definitely has a purpose and is capable of being put to use. And those 900 million active accounts must be getting some value or benefit by doing so otherwise they wouldn't do it.
So once again your proven wrong which is really astonishing because by now the law of averages would tell us you would get something right even if it was by accident. hahahaha
Something can be popular because it is useful or because it is entertaining.
FB is entertaining, people are amused by it.
They get to talk about themselves.
Other then that, it has very real use value, since millions of people live without it without any problem.
No one has to 'experience' FB to know that it simply a waste of time.
And I think many are coming to realize this.