Fediverse

I feel like in a lot of ways, app.net was ahead of it's time. It's whole goal was to build a "social backbone". A social account and network of linked users. On top of that backbone you could build out services, and all your friends were already there. In a lot of ways, it did what the fediverse is trying to do with Kbin, Lemmy, Mastodon, Pixelfed and activitypub in general.

I remember they started with a Twitter clone, but a clone for Vine, and Instagram soon appeared. It was wonderful while it existed.