I am trying to work out the strengths and weaknesses of messaging platforms from fully decentralized to federated to centralized. I am only a user on Mastodon/activitypub and IRC, but I have in the past hosted usenet and currently host:

smtp (email)
XMPP
Matrix
SSB (Secure ScuttleButt)
SIP (fully decentralized)

The impressive feature of twitter and it's totalitarian centralized ilk is that a single id can have millions of followers - and know that. SSB supports unlimited secure (signed) broadcasting, but there is no mechanism for knowing how many followers there are. Of course, TV was in the same boat, and you could get an estimate by polling. BBC broadcasts on SSB.

Counting followers is essential for monetizing content via advertising and sponsors in a decentralized manner - i.e. not subject to cancellation at a whim by a global centralized platform.

Matrix seems ideal for many of the purposes people use Teams or Substack or Slack. Private conversations e2e encrypted, logging with controlled retention (HIPPA compliant), voice and video calls, voice and video conferencing, media. But performance of small personal servers drops with number of participants in a room - I don't think it can support a million followers.

XMPP has inconsistent state for multiple devices, and is terrible at group chats. I do use it a backup for matrix and for voice/video calls. Open XMPP clients supporting VOIP and IPv6 are easier to find than SIP clients. (And SIP is even worse at state for multiple devices.)

Usenet has no authentication (not worth tacking on GPG header schemes).

Email is not designed to be "instant" (as in IM), but can be coaxed into resembling that by clients such as DeltaChat.

Ok, so now I should make a feature matrix (which includes Matrix), but have I missed any open and federated/decentralized protocols? Any other features? Current feature list:

broadcast (million+ followers)
follower count
p2p voice/video
e2e encryption
authentication
federated
decentralized (or federated that can be practically fully decentralized, like SMTP)

Did I miss any?

@customdesigned I don't rhink that matrix messages have any retention

@engelbart They are kept indefinitely. But you can set a retention limit on matrix-synapse - or as a user post a tombstone on a room pointing to a new, fresh one.

@customdesigned so, I'd say matrix sucks on this.

But I still love matrix for other features.

@engelbart What is "this" you don't like? That comments don't disappear unless you take steps? Or that comments can disappear eventually?

@customdesigned that I can't set retention period for my messages.

@engelbart Oh, but you can.

@customdesigned how?

@engelbart https://matrix-org.github.io/synapse/latest/message_retention_policies.html

@customdesigned seems not to be accessible on my homeserver

@engelbart What do you mean exactly? Are you trying to DM me? On what server?

@customdesigned Do you know what is a homeserver in Matrix?

@engelbart It's like a mastodon/chatter/pleroma/etc server but for Matrix protocol instead of ActivityPub protocol.

Protocol website at https://matrix.org, one list of homeservers at https://joinmatrix.org

@customdesigned Yes. My homeserver is matrix.org. And as I can see it doesn't support retention for the messages.
So I can't specify auto-delete timer in my groups/chats.