What I'd do is have a shared highfanout queue. Once you have more than say 10k followers your tweets go to the high fanout cluster. You'd have hundreds or low thousands of machines, each of which serves a slice of consumers. When the tweet is sent, you write it to this queue. Each consumer is pulling from one of the shards. If you have a thousand workers that means each worker only needs to send a thousand messages in three seconds, which is very doable.
Only about 180k twitter users have more than 20k followers. If each users tweets every 200 seconds, which seems like a high estimate, then that implies a load of about 1kwps for this system, which seems doable, especially if you have a small intermediate layer of distributors which consolidates the write load.
That's just my sketch.