ActivityPub separates content from platform. Posts from one platform propagate to other platforms, and users don’t need an account on every platform ... for ex., YouTube clone PeerTube and Mastodon both implement it, so is Mastodon user A follows PeerTube user B, B's new videos will appear in A's Mastodon feed. A can even comment on it from Mastodon
your ActivityPub identity is already there on all ActivityPub platforms, making impersonations impossible.
This flips network effects from the platform to the protocol: the more platforms adopt ActivityPub, the more valuable it becomes for more platforms to join in...
But it still faces the empty train syndrome, so funding is critical. And ads won't work: " There is no centralized audience to segment and advertise to. We’ll need to rethink the fundamental business model of social networking if we want ActivityPub to take off. "
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See also: Online Community Management