but this far, it hasn't. t reason has been surprisingly ineffective. >> some lies never really go away. these are not fringe ideas being promoted by fringe people who don't have an audience. these are fringe ideas being promoted by people very much in the mainstream. what i worry about is that people will start to believe that there are no good sources of information, and that they have no way of knowing what is true and what is not true. >> reporter: pressure continues to build on social media giants to intervene, and play a more active role in fighting disinformation. and in response, ceos from twitter, google, and facebook, have pointed to changes, including fact-checking measures and de-platforming users who spread conspiracy theories. >> some of you will say we're doing too much and removing free speech rights. some of you will say we're not doing enough and end up causing more harm. both points of view are reasonable, and worth exploring.