People say capitalism is efficient, yet Twitter has around 5,OOO employees while Mastodon was built pretty much single handedly by Eugene Rochko. Today, Mastodon provides a strictly superior user experience with only a handful of contributors.
Majority of effort at Twitter is directed towards things like ads and tracking that are actively harmful from user perspective. Meanwhile, the core functionality of the platform that benefits the users can be implemented with a small fraction of the effort.
Seems to me that capitalism is actually far more inefficient than open source development in practice.
I believe in capitalism, but trying to argue that Mastodon vs Twitter is not a good comparison seems like you want to cherry pick. I think the better argument in favour of capitalism is that it was the free market that allowed people to fund Eugene and all the other contributors to build the Mastodon network because that is what users wanted.