Yeah, we're used to Tumblr, and I think for many people it's the only site they have known for fandom (or at least the main one) as you say.

I agree with you about Pillowfort, I've heard that right now it's slow and not prepared for the amount of people that want it to be Tumblr's alternative, so in any case I'll wait and see. Maybe it fails at being that, I don't know. As I said in another comment, I don't see it improving fast enough for the hype to last.