for a long time i didn't really get tumblr, now i do... the reason it is really really good, and the reason it matters, is double blind distribution means that i can consume pure content without social overhead.
to explain. -- let's *pretend* i want content from jenna jameson.
facebook bidirectional friend relationships help grow their system, but also clearly structure and force me to curate my content and connections. friending someone has a positive value and a definite socially enforced cost structure. i don't want to be friends with jenna because i don't necessarily want her to have access to me and my information. if she did, it would negatively impact my ability to share high value information in the network.
twitter's unilateral follow is lower touch in several ways, but there is still enforcement. if jenna follows me, it isn't a big deal... because twitter is open, i fundamentally can't share anything on it that i wouldn't want on the front page of the wall street journal anyway... and she doesn't get much with her follow.
but if i follow jenna, that is a horse of a different color. i am giving information that i want her content/am consuming her content to others (like my mom) or i need to play where is waldo and follow tons of people (diluting my stream value)
tumblr is perfect for following jenna.. it is content without implicit social capital flow, only explicit gestures like reblog...
so, with tumblr i don't have to display who i am following, and she doesn't know i am following her!, and i still get the content in a nice streamed format.
i can use tumblr for pure content minus the social strings because it is double blind. making tumblr far racier... (and far more honest perhaps?)
the cool part is that this construct seems to play out - tumblr is about content, no strings attached - and so it is far racier (and has far more book deals)...
and, what is even cooler is that just because the tumblr blind graph isn't shown to users directly doesn't mean it doesn't exist or is worthless... it is actually super valuable data.
growth implications: the only issue is that double blind doesn't grow on the same dynamics, twitter and fb skim existing social capital to grow, tumblr doesn't - but with scores and reblogging, tumblr tries to short circuit these issues...
update: a few people have pointed out that you can get a list of followers on tumblr (i hadn't known that!) most of the above still works/applies because that list is private... but i guess jenna will know i am in to her after all (even if my mom doesn't know it)