After reading Sheldon's answer http://zhi.hu/PYKV
, I'm going to have to agree. The survey result is inconclusive and possibly misleading.
Facebook just passed the 1 billion user mark last Fall. A survey of 1000 kids can't represent the full picture.
Also, "Multiple answers were OK." So the data could be completely overlapping. If every Facebook user in that survey also used Tumblr, that means both are alive and healthy.
I think what's a better measure of popularity is the growth rate of each site.
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But if I had to think of a reason why I would even need Tumblr when I already have Facebook, I'd explain that they're not really the same social networking tools, just like how a hammer and a screwdriver are both tools, but have different functions.
One drawback of Facebook is that I have to be the same person to everybody at the same time. When I post something on my wall, all my 'Friends' will see it. I could try to exclude people and make groups and only post to different groups, but it's a lot of work and the interface wasn't designed for it. An even bigger problem occurs when I post something on somebody else's wall; then all of my friends and all of his or her friends will all see it, and I have no control over that.
This is troublesome because not all 'Friends' are equal. Some of my 'Friends' are my actual friends, some are my best friends, and some are really just old classmates. So when I'd like to post a nasty joke on my best friend's wall, I have to consider how our mutual friends, who might not know me as well, will interpret it and what they'll think of me or my friend. Add to that the family members and potential employers snooping around Facebook, and I can only present a socially acceptable, politically correct image of myself, not my true self.
Then we come to Tumblr. It's like the old days of the internet where your real world identity didn't mean anything to anybody unless you wanted to show it off (similar to Reddit). I can have as many Tumblrs as I want; one about my photographs, one about my serious thoughts, one about the dinners I cook, one about silly cat gifs, one about nerd memes, one about men's fashion and an extremely messy one that holds everything that doesn't fit into any of those categories.
And people follow my separate Tumblrs, they don't follow me. I can share a Starcraft joke in one Tumblr and a commentary on Mandelbroit's stochastic models on another Tumblr and expect appropriate responses on both. My world is neatly categorized.
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Yeah, those pictures of random girls on Tumblr - how do I get that out of my feed?