Last week, the market share of visits to Twitter surpassed Digg for the first time since launch and was ranked #84 (one above Digg at #85) in the Hitwise Computers and Internet category.

A big driver of traffic to Twitter last week was around the US Airways plane crash in to the Hudson River last Thursday, driving many posts and updates about the situation.

One major shift that has occurred is the increased traffic to Twitter from Internet users aged 25-34. During the same time frame last year, 12% of Twitter’s traffic was represented by those aged 25 to 34, which has increased to nearly 45% for the 4 weeks ending Jan 17, 2009.

Also interesting is Twitter’s clickstream, where Digg relies heavily on traffic from Google (38.8% last week) while Twitter receives a higher share of traffic from social networks, which is mostly due to the applications which integrate the services.

One caveat is that a significant amount of twitter activity takes place on mobile devices (especially since there are several twitter apps for the iPhone), which is not measured here, so the impact of Twitter is actually higher than solely web visits. Although this only tells part of the story, judging by the amount of tweets about today’s Presidential election, we’ll continue to see Twitter’s traffic increase.