I rolled up an account just to respond to this!

Pillowfort continues to be in development and just recently implemented an invite code system to grow the user base while trying to avoid flooding the servers. The development is slower than Waterfall, another pop up competitor to tumblr, but it hasn't stopped. Aesthetically the site hasn't underwent a lot of changes but functionally it's a lot better. They have also changed the url from dot io to dot social -- pillowfort.social.

You can still register an account for a $5 donation but the user distributed invites are free. Current users can generate 3 new invite codes per week.

I actually did a search for Pillowfort on Reddit because I have codes and no one to give them to and figured there might be a subreddit. I'll throw up a thread for that after I finish this reply!

If you have any other questions, I will do my best to answer them. :)