politics
Seriously, how about the bumper crop of amazing political videos this year, though? I'm a big fan of a get out the vote effort that actually engages people, and getting a video to go viral is a fantastic way to get people thinking about actually going to the polls and filling out their ballots.
climate change
In the course of making a fake bad social media campaign, the activists created a big social media problem for Shell, which was, of course, precisely the goal.