Source: http://www.forbes.com/sites/jeffbercovic...

"I think every company’s different, every founder’s different,” says Bijan Sabet, whose venture firm, Spark Capital, was an early investor in both social startups. “They’re two different situations. It’s a funny day when we look at $1.1 billion as small.”

Karp’s lack of interest in the mechanics of revenue was a big factor in winding up at Yahoo, Sabet agrees. “David’s true love is product and thinking about the user,” he says. “Now he gets to do that full time.”

Brand. It may not have the market penetration of Facebook, but every wired American knows Twitter’s little blue bird symbol. Do you know Tumblr’s logo? Probably not. “It hasn’t been able to break out to the mass market the way Twitter says,” says Mehta. “You can’t imagine your parents ever having a Tumblr account. That’s a big difference.”

It’s one that stems from who sets the tone on the network. Twitter undertook a concerted campaign to get actors, comedians, athletes and politicians using its service, notes Gartner’s Blau. “Tumblr did the same thing, but the people that self-selected Tumblr was the artist and designer crowd,” he says. “Those are not influencers. In fact, those are people who really want to be off by themselves.”

Content. In several ways, the nature of content that thrived on Tumblr made it a difficult business proposition. Some unknown proportion is pornography; estimates range as high as 30%. (One study found that 11.4% of the 200,000 most-visited domains were adult content.) A good deal more of it is copyrighted material, republished without consent. “I think Twitter is an important thought platform for the world, whereas Tumblr just isn’t — it’s a reblog site for ‘memes,’” says Tolles.

Don’t underestimate the importance of format, either. In the 140-character tweet, Twitter invented the equivalent of a new quantum particle — something that seemed like it should have existed all along. “They found a communications mechanism that really resonates with people,” says Blau.

Mobility. Twitter was born as an SMS application. The migration of the internet from desktops to smartphones was never anything but a good thing for its growth. Meanwhile, “Tumblr was very desktop and had to port to mobile,” notes Mehta. That posed various challenges. It’s hard to blog from a phone, and the big, gorgeous images that populate so many Tumblrs don’t look like much on a 3-inch screen.

Data: While Tumblr had a lot of users, on a per-user basis they weren’t nearly as valuable to marketers as the people on Twitter or Facebook, says Pakman. The nature of follow/friend relationships on those sites generates the kind of data advertisers crave — about interests, affinities, personal relationships. “In the case of Tumblr, it’s not really clear what the data tells you,” he says. “It’s not clear why people follow other people. It seems to be because you like their creative expression. I’m not sure that’s very valuable data.”

Geography. Karp has always said Tumblr’s New York City location was a crucial element of its success. Tolles disagrees. “It’s hard to make a platform work outside Silicon Valley. I just think there’s a set of resources that NYC doesn’t have that gets brought to bear — executive talent, technical talent and a desire to work together with other people in the industry.”