So Pillowfort is like a replacement for Tumblr that allows NSFW art and stuff?

Yep

I wish them luck! I never used Tumblr myself and from an outsider's perspective, all I can really see is that it's basically a platform like Twitter, but with more traditional 'blog entries'. Is that a fair summary? And yeah, I know many sites did this before Twitter did, but I never used Blogger, MySpace etc. either.

I don't know. I've never understood how to use twitter much, and I never really used tumblr or any other blog site. It's just another place where I can look at and share art.

Fair 'nuf!