But a common criticism of Nielsen’s study–which I also raised–was that Nielsen was only measuring visits to Twitter.com and not Twitter use via third-party apps. Theoretically, it’s possible to be an avid Twitter user without ever visiting the service’s Web site more than once.

Point taken, says Nielsen, which says it reran its study to account for that. And… no change! From Nielsen’s blog:

“As an update, we went beyond just Twitter.com, adding in more than 30 websites and applications that feed into the Twitter community including: TweetDeck, TwitPic, Twitstat, Hootsuite, EasyTweets, Tumblr, and many others. The results verified our initial findings: about 60 percent of people on Twitter end up abandoning the service after a month.”

If you’re so inclined, you can argue that Nielsen is still missing a big swath of Twitter users because it seems to have ignored mobile apps, which is where a lot of Twittering occurs.

But I still think Nielsen’s numbers sound about right, both in methodology and conclusion. I think it’s entirely possible that the majority of people who try Twitter don’t end up sticking with it, and I think that people who do use Twitter on third-party apps still end up at the site once a month. I do.

You owe me. I had to get off my sickbed today to call David Karp, the CEO of Tumblr, and ask him if he really was in talks to sell his company to Yahoo (YHOO), as you reported today on Gawker/Valleywag. For the record, David says your report is “categorically untrue.”

I’d spend more time discussing it, but, as I said, I’m feeling under the weather. And your story is not true. So here’s what Karp has to say via his Tumblr account.

And here’s what Marco Arment, who helped start Tumblr with Karp a couple of years ago, says:

And here’s what venture capitalist Bijan Sabet, whose Spark Capital helped raise a $4.5 million round for Tumblr last year, says via his Twitter account:

Now I’m going to return to my convalescing. Please give me a break for another day or so.

[Image Credit: Library of Congress via Flickr]

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]