The internet monitoring firm said visits to Twitter in September increased by 1,170 percent compared to year-earlier figures. Hitwise also said Twitter was the fourth most visited social networking site in the US, and grabbed 1.84 percent of the US market among the top 155 social networking sites in September 2009. That's a huge increase over the same month last year when the microblogging site only had 0.15 percent of the market, Hitwise said.
Market leader Facebook saw its already impressive market share increase by 194 percent, letting it easily maintain its recently attained place atop the US market.
Facebook, which grabbed its 300 millionth user last month, captured 58.59 percent of all US social network visits last month, compared to 19.94 percent the year before.
MySpace, while still easily in second place with a 30.26 percent share of the US market, saw the biggest slide year-over-year, reported Hitwise. Myspace share has plunged 55 percent from its September 2008, market leading 66.84 percent share.
Visitors to Tagged, a five-year-old social networking site, increased by 47 percent to take third place with a 2.38 percent share of the US market last month. And myYearbook.com rounded out the top five with 1.05 percent, 40 percent less than last year.
Both Facebook and Twitter have been on a roll for the past year.
Online traffic tracker ComScore reported in April that the number of Twitter's US visitors increased by 131 percent between February and March of 2009. Twitter had 9.3 million visitors in March - five million more than it did in February, according to ComScore.
And in June, Nielsen reported that Facebook users spent 13.9 billion minutes on the site in April, a dramatic hike from the year-earlier total of 1.7 billion minutes.
See also: Twitter to get 'video tweet' function