There seem to be a lot of ironies at play in this situation
Of course, this "message" should be protected as free speech, but it has more of a prankish/needling feel to it. "Let's put our finger in the eye of the majority and watch 'em get angry."
Or, is this just simple proselytizing? The Great Commission for atheists? Will atheist missionaries force their Western style beliefs on peoples of "simpler" cultures in an attempt to make them give up their belief in God/gods?
Or, is this just the annual membership drive?
Or, information for an atheist support group?
In the linked photo, I don't see where the billboard says "You're not alone." But, I'm sure many believers in God would find that answer to that question to be full of irony. Maybe that was the intent.