Maybe a social network that prioritizes high-quality content rather than fresh content. High-quality content (in terms of being e.g. persuasive & informative) may have a better chance of creating positive change than one's typical posts..

I'm working on an experiment at https://toplists.app/ that tries to leverage existing social networks, but this would probably work much better if e.g. Twitter expanded its "pinned" posts so that the UX would be seamless.

Also - perhaps software that improves visibility/transparency/understanding of energy usage, the lives of people in poverty, victims of human rights abuses, farmed animals, political votes, the impact of donations (Effective Altruism), etc