How big of an impact did Oprah’s segment dedicated to Twitter on last Friday’s show have for the microblogging service? Hitwise is out with the numbers today, and they are substantial.

Traffic to Twitter.com was up 43 percent compared to the previous Friday, and 24 percent from the day before the segment aired. How much of that was attributable to Oprah and not just Twitter’s own huge growth?

Hitwise notes that “the percentage of new visitors was 5% higher on Friday than the previous day and the average for March,” which means that much of the traffic surge was simply due to organic growth that the service has been generating, not to mention all of the other media buzz leading up to the appearance.

Nonetheless, a 5 percent increase in new visitors is significant when you consider Twitter’s most recent traffic numbers. The service reached more than 13 million people in the US during March according to the latest numbers from Nielsen Online.

As for Oprah, she’s closing in on half a million Twitter followers, though, her usage thus far has not nearly been as prolific as that of her guest on Friday: Ashton Kutcher. The actor will likely become the first person to hit two million followers, as his follow count has exploded to nearly 1.3 million as of this afternoon. It has been less than a week since he became the first to pass one million followers.

By Adam Ostrow