Think about it: When the microblogging service debuted, blogging was all the rage. Twitter was in its infancy, and the hashtag as a social media mechanism didn’t yet exist. Facebook still looked a lot like MySpace, though a recently introduced feature called “News Feed” hinted at something more. Apple wouldn’t introduce its iPhone for four more months, setting in motion a mobile device arms race that would give rise to Google Android and mobile-first communications tools like WeChat and Snapchat.

So it’s remarkable that Tumblr (YHOO) has managed to hang around for so long. That’s a credit to its design chops, which remain among the best in the business, and its insistence that it not ruin the core experience of its service with ads or other distractions. (That’s changing under its Yahoo ownership.)

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But lately, Tumblr’s felt a little bit like a ghost town. That’s not to say that it is, at least by some metrics—according to market researcher Global Web Index, Tumblr is among the fastest-growing social media outlets on the Internet, with an uptick of 94% in active users over the last year. (Estimates put the company’s user base at around 500 million, on par with Instagram but well behind Facebook.) But it’s still frustratingly the same, a broadcasting service in an age of conversation. You can’t feel the thrum of human interaction quite like you can on other platforms. Though its reblogging feature is as strong as a Twitter retweet, other participatory elements—multi-threaded conversations and the ability to react to them, for example—are missing.

“Follow the blogs you’ve been hearing about,” Tumblr’s login page proclaims. But what about the people?

Tumblr knows it has to keep up with the times, and the times dictate that Tumblr look beyond the blog. Two weeks ago, if you logged into the service you would have found the first-ever livestreamed NFL football game at the top of your feed. And starting this week, a lucky few—1,500—will see Tumblr’s first-ever instant messaging feature make its debut. (The rest of us will get it within a month.)

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You might miss the new feature at first because it’s a real-time twist on Tumblr’s existing “fan mail” feature, which replicated the asynchronous approach of e-mail. But that difference is important. Like Pinterest, Tumblr has historically proven to be something you could check on your own time. You didn’t open it as compulsively as Facebook because what it offered wasn’t timely or a necessity—just enjoyable. You weren’t missing world news or your friends’ reactions to it. You weren’t leaving someone hanging because no one was trying to use Tumblr to communicate privately. You weren’t missing someone’s birthday or a party invitation.

The introduction of instant messages take Tumblr a step toward more urgency. It may take awhile for Tumblr to train its users to think about the service in a new way. Adoption will be slow—attention-challenged Internet users certainly don’t need another instant-messaging inbox. But if Tumblr can make real-time interaction as enjoyable as its core service has always been? Well, then it might be on to something.

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