Tumblr is finally giving marketers what they asked for: the microblogging site announced today that it has teamed up with Union Metrics to provide an enhanced analytics platform enabling brands and marketers to measure engagement levels on the site.

Tumblr hosts more than 75 million blogs and each day users create more than 70 million new posts, according to a company statement. The company previously only offered metrics through Google Analytics. Using social media data provider Gnip, Union Metrics will access the entire firehose of Tumblr data to help brands quantify and track the site’s activity. The platform will be available on an invitation-only basis, but it will soon expand to the public.

The platform’s offerings include:

• Filtering capabilities to track blogs or topics
• Summary analytics showing overall engagement and trends over time
• Identification of the most influential contributors
• Analysis of posts and tags to surface the most popular content
• Individual post-engagement analysis

A far cry from the site’s previous opposition to advertising, today’s announcement highlights Tumblr’s increasing willingness to court marketers. David Karp, the 25-year-old founder of Tumblr, echoed Mark Zuckerberg’s early stance on advertising when he told the Los Angeles Times two years ago that Tumblr is “pretty opposed to advertising. It really turns our stomachs.” Like Zuckerberg, Karp has changed his tune.

Launched in 2007, Tumblr offers a simple interface that lets users type posts and upload photos on their blog, which they can personalize by choosing from a wide array of themes. In addition to making your own posts, you can “follow” those of other Tumblr users. You can also repost images and other content onto your own blog, which has earned Tumblr a reputation as a more visually oriented version of Twitter.

Brands, especially online retailers and fashion designers like Stefano Gabbana, Oscar De La Renta, Asos, and Net-a-porter, have embraced the microblog as a place where they can quickly post Instagram photos and other content.

Like Facebook, which announced today that it has reached one billion users, Tumblr is still working on its advertising strategy. Marketers, what do you think? Would you use Tumblr?