In the last few years, political commentators have highlighted that we live in a post-truth era, wherein the deluge of information available on the internet has made it extremely difficult to identify facts. Consequently, individuals have developed a tendency to form their opinions based on the believability of presented information rather than its truthfulness. This phenomenon is made worse by the economic policies of social media platforms, which often seek to maximize user engagement at the cost of everything else. In fact, many studies have concluded that the algorithms developed by platforms for this purpose often promote conspiracy theories among users, which eventually leads to a decrease in trust on democratic institutions among the users. In this work, we present a game theoretic model where social media platforms are reluctant to filter misinformation due to a fear of loss of revenue. Then, we design a mechanism that a democratic government can use to incentivize social media platforms to filter misinformation. Our mechanism presents an optimal level of investment for the government, in the form of tax breaks or subsidies. It also presents an optimal distribution of this investment among the social media platforms, to ensure that platforms voluntarily filter misinformation and prevent an erosion of trust in democratic institutions among their users.