In 2010, Twitter became a truly global phenomenon with 25 billion tweets, some of them quite powerful. But most of its growth came form overseas. If you look at a comScore chart of U.S. visitors to Twitter.com (see above), it looks like Twitter’s growth in the U.S. is stalling. In November, Twitter.com drew an estimated 24 million unique visitors, down from 25.1 million in October. Unique visitors