I wouldn't really call it a protocol. If Twitter is a protocol, then every exposed JSON-emitting API is a protocol - and perhaps that's true. But Twitter isn't unique in that respect. People have been publishing messages without specified recipients for a long time - it's called broadcasting, or publishing, or web publishing.

I wouldn't say the recipients are totally unspecified, either.

What makes Twitter unique is the artificial limit imposed on messages that can be broadcasted indiscriminately. And that makes it more of a new form of communication than any kind of protocol. It leads to a new form of expression, and a new way of getting a fairly accurate global view of the attitudes and cultural tides that are flowing through humanity in real time. That does make it truly earth shaking.