They use Facebook as a utility. They check Facebook when they wake up and check it before they go to bed. But their profile on Facebook looks just like everyone’s profile.

A Tumblr is self expression. Jessica’s looks different than Emily’s, mine and the Gotham Gal’s. That’s powerful. And that is what I think is driving Tumblr’s popularity. Self expression matters.
— Fred Wilson (2010) Self Expression Matters
Self expression is just an external observation of a more psychological dynamic: identity.
I’m saying this since MySpace started to raise: identity expression is a critical element in any social networks. That’s why it’s one of the four elements of Social Usability.
Just to give some more pointers:
  • MySpace: gets it, and it was the biggest selling point… and still is. That’s exactly why still millions are using it.
  • Facebook: completely misses it. But that’s what they want: they keep saying that Facebook is a development platform. So be it.
  • Twitter: gets it in a simplified way.
  • Flickr: misses it. If you’re wondering why so many personal pages services for photographers exist today, the answer is simple: Flickr isn’t providing any customization.
  • DeviantArt: misses it.
  • (you can grow this list as much as you’d like)
Of course, I’m not saying that you must build an identity feature in anything even remotely social you build, I will be as wrong as the ones not getting it. Instead, you have to think if you need it. But that should be a negative answer to an understood concept, not complete ignorance.
And of course, the Social Usability Checklist exists to help you in the process.