This guy says that Twitter is getting spikes up to 11,000 page requests per second.
That works out to a peak load of 950 million pageviews/day That is as much traffic as facebook did per day in Jan of this year. In Twitter’s case probably 99.9% of those pageviews are RSS, automated requests and status polling of some kind.
This company reminds me a lot of Youtube, lots of bandwidth and no way of monitizing it. Twitter is going to have a tough time covering 6 figure bandwidth costs if those numbers are even remotely correct. But at least for twitter they have traffic, and traffic gives you options, such as acquisition!