Now, some would say that, these services had the goal to make money and that's why they attracted capital and could hire top talent, but it doesn't change the fact that they had to have a strong product mentality to get to where they are today.
Most decentralized software I see have a project mentality based on idealism, but idealism is not enough. That's why desktop Linux failed to capture significant market share even if it costs zero to Joe Average to install it: it stayed at the level of idealism, of a project, while proprietary platforms like Windows an OSX had serious money and talent behind them because they weren't just projects.
On the other hand, Firefox and the Linux kernel are good example of successful open source softwares with a strong product mentality. Mozilla had a vision and they realized they need a strong product to fulfill that vision, so they needed to be able to pay their developers well, meaning they had to generates money. Same thing with the Linux kernel, which is backed by a lot of big companies.
So I guess what I'm saying is; if ActivityPub is to be the future, then there should be more than idealism behind it.