The rise of the popularity of a platform does not imply that those who migrate to it en masse grew more numerous, only that they prefer this particular form of expression.

Groupthink is a thing. Emergent communities are a thing. Tumblr isn't just a publishing platform, it's a social platform, and that makes it meaningful to talk about "Tumblr" as a social entity rather than a purely technical tool. And when looked at in that light, Tumblr (the community, or more specifically the radical politics community as opposed to e.g. the catpic community) almost certainly radicalized many people who wouldn't otherwise have gotten involved; naturally there would have been other centers of radical politics and other vectors of radicalization if David Karp had been hit by a bus back in 2004, but that's not the world we live in. Similarly, MySpace played a role in the rise of indie music by exposing people to it through social-media channels.

We're typing this on Less Wrong, dude. I can hardly think of a clearer example of online movement-building. Do you really think that this community -- movement, cult, hugbox, whatever you want to call it -- would look much like it does now if Eliezer had stayed on the SL4 list, or gone straight to publishing a book?