There were plenty of stats doled out by Twitter's founders during Chirp Conference keynotes today. The two that stuck with us were: 1) that its servers handle 3 billion calls every day, just to the API, and 2) that 75% of all their traffic comes from their API. If you look at the volume alone, that's over 30,000 updates, timeline requests and searches per second. That's a massive API.
That other number, that 75% of Twitter traffic comes from third-party applications, demonstrates just how much of their growth and popularity rides on the efforts of outside developers. It reinforces just how important the platform is to Twitter. As Twitter mashups grew, so did the use of Twitter.
Even back in 2007 we wrote that Twitter's API traffic was 10x the Twitter site. The exact percentage via their site or via their API varies over time, but it's clear that the API has driven, and will continue to drive, a huge percentage of Twitter's traffic.
And, according to Twitter CEO Evan Williams, engaging new users is the responsibility of Twitter, not developers. The expectation is that the rising tide will lift all boats, as it has through the tremendous growth Twitter has seen.