I recently came across this post from May 2008 comparing Twitter traffic and the Options Price Reporting Authority data feed. Needless to say, the stock market feed is many orders of magnitude larger, at 700,000+ messages per second(!)
It's also not the fairest comparison in the world on its face, for a variety of reasons: the OPRA data system was planned (Twitter met success more or less by accident), Twitter is minimally funded, etc.
A more relevant comparison, in my opinion, is that provided by newzwag, which presented its performance challenges, triumphs, and secrets at a recent SF Ruby meetup.
newzwag's site and trivia game is built on Rails, started small, and had to grow to meet traffic driven by Yahoo and the Beijing Olympics to 9 million pageviews per hour (using a total of a half-dozen machines or so). And lest you think this is a content site served out of a cache, most of the traffic consists of data writes by game players that then need to be ranked, published, etc.
As far as I can tell, that's somewhat larger than Twitter, even considering that Twitter has grown 3-4x since last May's stats.
newzwag's solutions, which they share here, are a study in sanity, reasoned problem solving, and smart efficient architecture.
Without the timelines or resources of a stock-market app, newzwag produced a nice solution that -- at least in hindsight -- appears drama-free.
Interestingly, a newzwag - Twitter comparison can be enlisted to support a variety of different startup social narratives.
One narrative is that an amateur-hour effort yields amateur-hour results, and aspiring startups shouldn't fool themselves into thinking that they won't need old-time Architecture and Sophistication to scale.
A different narrative says it doesn't matter -- if Twitter's success is your worst-case scenario, you still win. That is, build it fast, get it out there where people can try it, and you should be so lucky as to need a real re-arch to fix your scaling problems. In this model, both Twitter and newzwag played it right -- newzwag because they knew the Olympics would provide a narrower time window to showcase their system, so they managed risk against that stricter goal.
And yet another narrative says if you accept these two stories, you still wouldn't want your brokerage transaction flowing through a system built to "see what sticks," and hence Web 2.0 startup methodologies stare at mission-critical business apps from across a huge chasm.
I see this last story as persuasive but also as a big opportunity: there is a chasm, to be sure, but it needn't be quite so big. There are legacy mainframe apps that can speak webservices. Every manager in a big company wants their product to be "100% critical" even if they could create more value by admitting that a lot of nice-to-have two-nines business apps are the real bricks in the wall. If enterprises can get better at separating their Twitters from their OPRAs, they can make more money and have both.