Randall Munroe, creator of the popular web comic XKCD, first made the observation with his own chart. Munroe's not the only one to note Tumblr's ascendance. ComScore data show Tumblr got 21.8 million unique visitors in the U.S. in March, compared to 8.4 million in March 2011. Despite that surge in traffic, Tumblr CEO David Karp told Advertising Age last week that he has no desire to monetize the p