Instead, the New York-based company has just raised a $4.5 million Series B round that its CEO, 22-year-old David Karp, says will fund it for two and a half years. Union Square Ventures and Spark Capital, which led the company’s first $750,000 round a year ago, also led this financing. My educated guess is that its investors now value Tumblr at around $15 million.

Tumblr is a “microblog” platform that is supposed to let its users quickly create nice-looking posts with a minimum of effort; it sits somewhere between Twitter and full-fledged blogs like Blogger and TypePad. It is popular with a relatively small but prolific user base: Its 500,000 users have created pages that draw 15 million unique visitors and 61 million page views a month, Karp says.

Money? Nope. Tumblr is free–and has no ads cluttering up its hipster vibe.

Way back in 2007, those kinds of numbers would have been catnip for the likes of Google (GOOG) or Yahoo (YHOO). Now, though, even deep-pocketed buyers aren’t racing to snap up revenue-free start-ups. But Karp says raising money from his previous investors was easy: “These guys came to us with a deal that made us incredibly comfortable.”

Karp says he’s going to start generating money in early 2009. Not by selling ads on his members pages–Karp thinks the site will eventually incorporate ads in some way, but not yet–but by selling users “premium” services, most of which he’s not ready to describe. He does promise they will be “really sexy.”

More practically, Karp points out that other Web services, like the WordPress blogging platform and Yahoo’s Flickr photo service, have been able to upsell many of their users with goodies like extra storage. He figures many of his users will pay up, too.

The new money means he’ll have time to prove his thesis. Karp has just doubled his staff–which means there are now all of six people on payroll. One of them, hired in September, is John Maloney, Karp’s former boss at Urban Baby, where he started his Web career. Maloney is now in charge of business operations.

The money also means there are heightened expectations. Karp has done much more than people twice his age (and has the press clips to prove it). But the newest funding round means his investors think the company will be worth as much as $50 million by the time he sells it or raises more cash. In order to prove them right, he’s got a lot of work ahead of him.

[Image Credit: David's Log]