tl;dr:

1. Until recently, many brands have dismissed Tumblr as a place for kids to post cat GIFs, with little opportunity as a platform to effectively reach their customers

2. Consumers in the critical 18-34 year old demographic are hooked on Tumblr, with the average visitor spending 1.5 hours per month on the site.

3. Today, Tumblr hosts 90 million blogs that generate more than 89 million new posts every day.

4. This astronomical growth helped the site soar into lists of the top 10 most popular websites in the U.S. at the end of last year and is attracting significant interest from brands ranging from fashion to finance.

5. "Tumblr is a highly visual experience, so brands can appeal to us on a visceral level through stunning images or animated GIFs that capture brief, emotional moments. This kind of visual storytelling has been missing from digital advertising, but is what we’ve come to expect from the best TV ads. Tumblr makes that kind of brand experience finally possible online."

6. "A typical Facebook post gets 18 hours of engagement, while a Tweet only gets about 18 minutes... Social amplification and tag-based discovery are core to the Tumblr platform, giving content staying power, with many posts receiving reblogs and likes days, weeks and even months after the original post. A good example of this is a photo post from comedy website Funny Or Die – “21 More of the Best YouTube Comments” – that generated more 14,000 notes over several weeks, with only 3,500 of those notes occurring in the first 24 hours."

Also, Tumblr lets brands tap into micro communities:

Facebook connects people with friends and family. Twitter shows everyone what’s happening right now. LinkedIn helps people engage with colleagues and professional acquaintances. Tumblr harnesses the enthusiasm around a particular topic and coalesces into a community.

For brands, this simplifies the process of audience segmentation, because Tumblr provides a built-in audience that has already opted into following specific tags and topics. In turn, brands can post highly targeted content with interest-related tags, and easily reach communities of fans.

For example, Yale University wanted to connect with people interested in science and health, so they introduced a Tumblr campaign to coincide with theBig Food exhibit at the Yale Peabody Museum of Natural History. Based on data from the exhibit, they created two infographics – “How much sugar does your drink contain?” and “How much is 1 portion?” – for National Food Day. They posted both to their Tumblr account and included tags like food andnutrition to target the appropriate interest groups. With this highly targeted content, the university increased followers to its Tumblr by 142 percent and generated 1,200 notes across the two posts. It was also able to break through to health and fitness based communities.

I agree with the article's conclusion:

For much of its life, Tumblr has been a page view machine with plenty of potential. In order to realize that potential it still needs to continue working on uptime, grow its burgeoning team of industry evangelists and refining its paid offerings