Driving Tumblr's popularity: Self expression matters
avc.com
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Tumblr wins because it turns ideas into things and makes them easier to talk about. Not only this, but it also makes it really easy for your writing to get traction with others, assuming it's reasonably interesting. It's like training wheels for future intellectuals, a perfect match for intellectually curious kids in their late teens and early twenties. (In fact, it almost invents a new category of online interaction that I'd call 'intellectual sociability'.)
This was my analysis of Tumblr a year ago, and I think it still holds perfectly today: http://news.ycombinator.com/item?id=961523
Tumblr is more like blog+facebook+twitter, made ridiculously easy; minus the clutter and noise.
But the issue wasn't that MySpace was an 'eyesore' or that Facebook was 'clean', except to the extent that MySpace was full of distracting ads. Facebook is full of stupid crap, too. Facebook made the better choice to focus on innovating community features, which was its strength, where MySpace failed to innovate along its own strengths. Maybe was poorly designed or poorly managed, or maybe they simply panicked and tried to be facebook but clearly not as effectively. I don't know. But now it's a mess.
Facebook was and is a better tool for interacting with your social network than MySpace, and continues to innovate in that space. MySpace was, at one time, better than facebook at self-expression, media sharing, and making connections with semi-random strangers. Now, I'm not sure it's really good at anything particularly interesting.