I think there are issues with hotspots. The most popular tweets are seen by a big chunk of the userbase, which means they have to operate on a fanout model wher

I think there are issues with hotspots. The most popular tweets are seen by a big chunk of the userbase, which means they have to operate on a fanout model where each tweet is pushed to individual followers. I believe IRC doesn't operate like that. The messages delivered to each user don't need to be retained, and I assume the size of the largest channels is in the tens or hundreds of thousands.