First, I think that "they" is a term.... Social media platforms operate really differently, but, in general, social media platforms vary in how aggressive they are at de-amplifying false claims and amplifying other kinds of claims.
As an example, in the United States in 2016, a high percentage of things that were not true were exposed to people over Facebook. Facebook was then deeply criticized for that, and in 2020, that percentage dropped precipitously because Facebook engaged in more aggressive diminishment of inauthentic behaviour on its platform. When the criticism died down, it stopped doing that and that percentage is increasing again.
They're not static in how they engage in these kinds of behaviours. It often is in response to criticism and perceptions of potential regulation.