I don't think the point is to replace Facebook&Co though. It is specifically a tool developped for political activists. Everybody can use it of course, it is free software. Given that context the decentralization feature is most definitively a crucial feature.

https://joinmobilizon.org/en/faq/#facebook

And even more broadly, the whole point of Framasoft is to offer tools to use the internet in a decentralized way. This is not limited to this particular tool, they have many other project. All offering an inferior user experience than the existing solutions by the usual FAANG giants of course but again the point is decentralization and freedom.