What? It says "App.net is a real-time social feed without the ads." right at the top of https://join.app.net, and "Continue to join.app.net, our Kickstarter-like project to create a real-time social feed." right on app.net.
How is he supposed to tell you what it is if you don't even go to the website?
I clicked the link in his blog post, which took me here: https://alpha.app.net/. You're right though, 3 paragraphs into the blog there is a link to join.app.net.
How is he supposed to tell you what it is if you don't even go to the website?