It would seem that it is rather difficult to get people to unite and organize because of a common belief in...nothing. Nihilism tends to negate the belief in anything like social conscience and helping others.
What would their rallying cry be? “ C’mon, I don’t believe in a Higher Power, neither do you, so lets spend a lot of time and money and sweat helping in this time of crisis because, well, its the right thing to do! Wait...there is no ‘right and wrong’. Nevermind.”