Assuming it isn’t a rethoretical question : Mastodon is a micro-blogging platform (A twitter alternative) which was the first “big” platform to use Fediverse/Activity pub to decentralize (I am not sure what was created for mastodon and what was already there).

And indeed, you can follow lemmy communities from Mastodon (but the UX isn’t great) or post to lemmy from Mastodon