Hah, you're right, you end up in some slow-loading dashboard (hooray fancy graphed realtime numbers!). I clicked on a random hostname, it's another dashboard page with the hostname in large, the same hostname as a link as a subtitle. I clicked the link, and hey, it's a page that looks like Instagram's sign up page, with the difference being that this one says:

"Registrations are closed."

I tried a few more sites, even the one with the most number of users, and they all say this. Only the 5th site I clicked on allows me to register.

Ah only now do I notice the column "Open Signups" in their fancy dashboard...