In the lobby of the Tumblr offices in New York, a young blonde receptionist in flip-flops is at work behind an enormous Mac monitor. When David Karp walks in, he has to sidestep the Rock Band instruments that are set up right in the middle of the office.

He looks alarmingly young, even for a 21-year-old. He extends an octopus arm and oversize hand and, sounding like a very polite teenager, says, "Hi, I'm David. Thank you so much for coming."

Karp is the founder and CEO of Tumblr, a software platform for tumblelogging. If a blog post is like an essay and a tweet is like a haiku, then a tumblelog is like stream-of-consciousness poetry. Users—upwards of 350,000 since the site launched—post bite-size thoughts, short videos, and candid pictures in a free-flowing style that, according to Karp, lets followers "see through the author's eyes rather than parse their editorial." Karp didn't invent tumblelogging, but his clever design has made Tumblr the preferred brand name among the online-networking elite.

When he was 17, Karp moved by himself to Japan, where he worked remotely for an American Internet company that knew neither the age of its IT guy nor the fact that he was telecommuting from the other side of the world. In February 2007, back home in New York, Karp designed and built the software for Tumblr in two weeks.

In Karp's office, on a white couch behind the desk, is his friend Charles Forman, 28, the founder of the gaming website iminlikewithyou. Like Karp, Forman left home at an early age. When he was 18 he headed from suburban Chicago to Korea and then to Japan, where he worked various programming jobs until he figured out how to combine his love of video games with his desire to be a Web magnate. On iminlikewithyou, social networking takes place via three-minute video games played in real time with online friends. Asked how he and Karp met, Forman deadpans, "Internet dating."

When the Internet boom flatlined in 2000, nobody anticipated that it would spring back to life a few years later, defibrillated by a demand for ways to socialize and self-promote instead of places to work and shop. MySpace, Facebook, YouTube, Flickr, CollegeHumor, Digg, Drop.io—these sites were built by and for people who want to construct online identities for themselves by sharing first-person accounts of their social lives and forming huge pools of acquaintances. And unlike the first time around, the power players weren't just geeks gone rich, they were geeks gone famous. Guys like Mark Zuckerberg of Facebook became bona fide celebrities. By mid 2006, Jakob Lodwick of CollegeHumor and Kevin Rose of Digg were kings of the new online social scene, their blogs and twitterings followed by thousands and their every move covered by online tabloids like Gawker and Valleywag. Karp and Forman, and their West Coast counterparts like Pete Cashmore, founder of the social-networking site Mashable, represent the newest iteration of the tech star—and possibly the saturation point of Internet fame. For them, aggressive online self-promotion is as natural as text-messaging—and as much a part of the business as software development.