And all the mainstream messaging services to date have been centralised, at least in the sense that they involve interacting with a third party server.
When each friend can be both a client and/or a server, no third party servers are necessary. In theory (and practice), this is something you can achieve on a small network consisting only of your friends.
What if all your friends want to be online at the same time?
What if they want to share photos and video while online at the same time?
What if they want to play games with each other while online at the same time?
You can currently do these things with the mainstream web-based services like Facebook. But they are recording everything you say and do _and_ selling that information for profit. You don't receive any portion of that profit.
Is everyone OK with this?
It's an open question, I guess.