Twitter is, of course, the popular microblogging service that allows you to tweet your contacts, in 140 character messages. Digg is the well-known and hit content aggregation site.
You can see from the Hitwise graph that while Digg has been fairly consistent in market share, Twitter has been going up, up, up, and it's just edged past Digg in the latest numbers.
As Hitwise notes:
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 photo of the plane taken by Janis Krums, was viewed by many people via Twitter and was subsequently used across a number of traditional media outlets.Hitwise notes that while social networking sites are a major source of Twitter's visitors, Digg relies on Google for much of its traffic. In fact, last week 38.8% of Digg's traffic came from Google, while 16.38% of Twitter's traffic came from MySpace / Facebook, and only 9.23% from Google.
Of course, this is a weekly pop for Twitter, as according to Compete, Digg had nearly 35 million visitors in December 2008, while Twitter had only about 4.4 million. On the other hand, as Hitwise states "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."