Tweetmeme is on a tear. According to Compete, the Twitter-centric link tracker went from nowhere in February, 2009 (with 26,000 unique visitors) to more than tenfold increase in March (to 385,000 uniques). Nick Halstead, The CEO of UK-based Fav.or.it, the company behind Tweetmeme, tells me he is tracking closer to 200,000 uniques a month based on yesterday's visitors, but that he is adding 50 percent a week.