Techcrunch and Mashable are both reporting that Tumblr has raised an additional $4.5 million in a Series B round with Union Square Ventures and Spark Capital. This was a surprising announcement in these troubled times however this invest will play a critical role for the platform’s future.

Tumblr has been gaining traffic and the funding will provide additional runway. More importantly though is that similar to Twitter, Tumblr lacks any form of a business model. Along with the funding announcement, founder David Karp has announced plans to launch premium services early in 2009. There is still no word as to what these services will be, however, my hope is that they are the “big features” first hinted at in October.

From the Tumblr blog, “big features” were announced back in early October with the last interface update. Since then the team has only delivered the ability to “like” posts and a few API changes. Neither of which justify the interface overhaul.

To this point, Pownce has really been the only other player to leverage the premium business model. Upgrading a Pownce account to the pro version added the ability to store more files and provided an additional level of design customization. However, one of the benefits of using Tumblr is its flexibility. Users can already customize the HTML and add custom CSS. And honestly, with Pownce closing its doors today, I wouldn’t bet the house on that business model.

It will be interesting to see what additional features are going to be added to the mix from a platform that made its name from being simple to use and extremely customizable. It sounds as if Tumblr will be hitting the ground running in 2009 and we look forward to providing you more updates as they emerge.