Tumblr is growing. Fast. Over the last 5 months, the social blogging platform has grown twice as fast as rival WordPress.com, as both services near the 20 million hosted blog milestone.

I should note that by “hosted blog,” I am referring to blogs that are accessed by visiting the WordPress.com website — WordPress also provides open-source software (through WordPress.org) that anyone can use to power a blog on their own website, but those blogs are not being taken into account here.

The number of blogs being hosted on WordPress.com has almost doubled over the last 12 months, from 10 million to 19.9 million. That’s a terrific level of growth by most standards.

However, Tumblr — which unveiled a new social sharing button Monday — is blowing WordPress.com’s growth out of the water. Take a look at these charts put together by Royal Pingdom, which compare the number of blogs hosted by WordPress and Tumblr in Dec. 2010 and May 10, 2011.

The difference between the services is around 1.5 million blogs, and that gap is narrowing by the day. Since December, Tumblr has grown twice as fast as WordPress.com, at least in the number of blogs each service is hosting. That is impressive, and it looks likely that Tumblr will overtake WordPress.com in the coming months.

Tumblr’s rapid growth is perhaps down to the fact that it is more inherently social than the WordPress platform. Where WordPress is designed for a more “traditional” style of blogging, Tumblr makes it easy for users to follow each other and reblog content to their followers that they find interesting. However, Tumblr has some issues when it comes to handing the sheer volume of traffic it is seeing. In fact, on Monday, I had to refresh some Tumblr pages 10 times to view the posts — not cool.

[Image Credit: flickr, chopan]