"And this community wasn’t imported from another network like a facebook’s graph. It was built purposefully for the Tumblr experience."
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This answers the One Network To Rule Them All question for me. I’ve always said that I like keeping my “graphs” separated (SoundCloud, Instagram Tumblr, Twitter, Foursquare, etc) and for that reason I find myself participating in a variety of other networks, ten times more than I do on Facebook.
Tumblr has enabled me build my own personal community. I’ve found more value in seeing strangers become friends rather than adding people I last saw 15 years ago in school as friends.