I keep feeling frustrated as valuable knowledge for my different hobbies over the last years became siloed away in corporate social media. I believe wikis could be a way out, but can we have decentralized, federated wiki software that can kind of talk among each other?
Wikis serve mainly lurkers, and federation of these sites does not matter much for them. The main advantage of wiki federation would be ability to edit several wikis under the same account. However, you can achieve the same effect with OAuth (that is, logging to many sites with the same account on another one).
And even that is unnecessary if the wiki in question allows anonymous editing.