Many open source, non-profit, social media sites exist. one of the reasons for-profit sites thrive is due to their profit incentive that ends up making them more engaging
Or.. that there cannot be thriving parks and beachs. There provably are. There are also lousy ones overrun with trash that no one likes or uses.
So the issue would seem to be - how best do we make the open source, public funded options more like the parks and beaches that everyone uses, and less like the ones that people hate...without a profit motive.