In this paper, we look at how social media, in particular Twitter, are used to trigger, propagate and regulate opinions, and social controversies. Social media platforms are displacing the mainstream media and traditional sources of knowledge by facilitating the propagation of ideologies and causes championed by different groups of people. This results in pressures being brought to bear on institutions in the real world which are forced to make hasty decisions based on social media campaigns. The new forms of activism and the public arena enabled by social media platforms have also facilitated the propagation of so-called “post-truth” and “alternative facts” that obfuscate the traditional processes of knowledge elaboration which took decades to arrive at. This poses serious challenges for Knowledge Organization systems (KOs) that the KO community needs to find ways to address.