Spam user registration

Hi,

I use WP on dozens of sites and just tried 3.8 for the first time. My site was online at 16:55 and 4 minutes later I got 334 spam user signups, even before I could do anything.
I use a safe VMware setup so the server isn’t the problem. How do they know that I am online on such a short notice…never encountered this before. Does this have something to do with the new version and possibly a bug or something?

Thanks
Ruben