Like many a Web 2.0 entrepreneur, David Karp hopes his product will be used by millions of people one day. But unlike other ambitious founders, the 21-year-old founder of micro-blogging tool maker Tumblr Inc. isn't planning an expensive, massive marketing blitz to grow the user base from today's 170,000 to one million people by year's end. Rather, Karp is focusing intently on satisfying his core a
