Microblog Memes

Same, but they're two very different animals, I think.

Bluesky's protocol, ATProto, was designed for rapid growth via centralized architecture (a few very expensive nodes) while ActivityPub (e.g. Mastodon or Lemmy) was designed with decentralized architecture (many inexpensive nodes). ActivityPub is less expensive since a message you send is treated like email in that only your home server and the recipient's home server must see the message; whether other people see it depends on who follows and/or replies to whom and who is blocked by who. In contrast, a Bluesky message you send is treated like a radio broadcast: by default, your message is publicly transmitted to every server without regard to who follows or blocks whom. Therefore, the minimum storage and bandwidth costs scale very differently: an ActivityPub server scales as a function of how many accounts its own users follow have while an ATProto server scales as a function of how many total users exist globally.

The benefit of centralization is the ability to reliably and quickly convey messages from all users to all other users by simply storing all messages then filtering through them when a user asks for an update. Also, moderation is easier, in theory, since fewer nodes must be monitored and regulated.

For details, see this post by ActivityPub developer Christine Lemmer-Webber.

https://dustycloud.org/blog/how-decentralized-is-bluesky/