If someone is following thousands of people in their stream, and they are scattered across a hundred servers, how do you go about assembling their stream for them in real-time. Is the host server going to have to send user id's for everyone every user based on that server is following, to every server where the people being followed are based, and have every one of those server send all the messages of interest so the host server can build streams for all its users?
Twitter didn't stop sucking reliability wise until they switched to a single persistent user stream connection and abandoned REST most of the time. You really want to have that one persistent connection for a user's stream.
That's a reasonable objection. However, could you get an aggregating server that collects the updates between networks? You might need to batch it, but for networks with better network connectivity it could be done in real time, or close to it.
There is no real need for instantaneous updates, unless the networks so deem it necessary.
Twitter didn't stop sucking reliability wise until they switched to a single persistent user stream connection and abandoned REST most of the time. You really want to have that one persistent connection for a user's stream.