Tumblr announced today that it has become one of the top ten sites in the U.S. and has a worldwide audience of 170 million people (based on Quantcast data). The 170 million appears to be rounded up from 168 million, but still, Tumblr is seeing tremendous growth.

Tumblr writes in a blog post, “This isn’t meant to be a brag – though you should definitely try our new Android and iOS apps if you haven’t yet – but a credit to all of your brilliant creative efforts. You guys have earned an audience bigger than the biggest TV networks, and you continue to make some of the most thought provoking, insightful, inspiring, funny, diverse, and compelling content being created today.”

In an update to the post, Tumblr says it was actually bumped up to the number nine spot in the U.S. after a data refresh.

In comScore’s Media Metrix for October, released last week, Tumblr was listed as the number 40 web property in the U.S. , just behind BuzzMedia (and ahead of Technorati Media and MySpace).

In January, Tumblr was at 120 million monthly users, based on Quantcast data.