Tumblr has a mobile problem. Unlike Facebook, Instagram and YouTube—Tumblr doesn't rank among the top mobile apps. The majority of the massive amount of traffic to Tumblr pages occurs in browsers, mostly on desktops. Not only is mobile traffic low, but because people aren't using the app, they can't be reached with paid promotions.
(Read More: Yahoo to Buy Tumblr, Vows 'Not to Screw It Up' )
Buying Tumblr would set Facebook back by 365 days. It would need to immediately retrofit Tumblr with a scalable mobile business model. And it wouldn't be easy. Tumblr is where you go to find content on Tumblr. Twitter is the best app for real-time content discovery across the entire Internet, including Tumblr content.
Instagram was acquired for its young mobile users, but also because it fit very well into the Facebook ecosystem. The majority of Facebook engagement revolves around photos, and Instagram images slide perfectly into the news feed.
They're ready-made status updates—pretty photos are how we communicate where we are, who we're with and what we're eating. Instagram images also make for ideal native ads, receiving a lot of engagement, reach and driving e-commerce conversions.
Tumblr posts typically contain titles, copy, links, embedded content, multiple photos and GIFs. These posts cannot be easily pushed into the newsfeed. Facebook wants to be the Internet, and Tumblr can't feed the animal what it wants to eat. Actually, Tumblr has similar ambitions for its own feed, dreaming to be the "the ultimate creative canvas" of the Web.
(Read More: But Wait. Didn't Yahoo Try a Deal Like This Before? )
There are countless artistic, entertaining Tumblr blogs. As Digiday Editor Brian Morrissey pointed out, they include content creators like marriedwhiteguy.tumblr.com; digital publishers like College Humor and themovieloft.tumblr.com; and traditional publishers including Newsweek, The Atlantic and CNN. Then there are all the brands. And the porn. Lots of porn.
Herein lies the biggest fundamental difference between Facebook and Tumblr: Facebook is based around true identity. It's your real name, location, friends and profile. This is the foundation of the social graph, keeping friends and family on Facebook together for so much time. Facebook is fueled by real people, not blogs managed by real people. It's hard to serve a native ad to a blog.
Yahoo was rightfully desperate for relevance on mobile and social. Facebook already dominates both at once. It doesn't have a need for Tumblr, because it doesn't fit into founder Mark Zuckerberg's blueprints.
If you already built it, don't buy it.
Jason Stein is the founder and president of social media agency Laundry Service, and a partner in Windforce Ventures, a VC firm focused on social and mobile. Stein owns shares of Facebook. Tweet him