On April 17, the day of the show--when CEO Evan Williams appeared on-air and Winfrey herself began Twittering--a whopping 37 percent of visits to the Twitter.com home page were new visitors. That's a high number even for the fast-growing Twitter. Keep in mind that many avid Twitter users rarely even visit the home page, instead relying on desktop- or mobile-based third-party applications, so the jump might not look quite as drastic if you factor all those in. But it's still high: Hitwise says that in comparison, recent counts of new Facebook visitors as a percentage of overall traffic are more like 5 percent.
There's nothing particularly surprising about this, as the "Oprah effect" has been known to propel books to the top of Amazon's bestseller list almost instantly. It's more surprising that Twitter's servers, once notoriously volatile, stayed on top of things for the most part.
But what's really going to be interesting is to see how many of these curious new visitors come back regularly.