According to Isnoop (Ian "isnoop" Anthony), they disabled the service as Gmail team felt the service was no longer tolerable ??
I have gotten the word from Gmail's Product Manager that my service is no longer tolerable. At midnight PDT June 7th, 2005, this service was disabled. .The Insoop service sent out 1,240,162 Gmail invites while it received 2,497,681 invitations from GMail account holders.
....Google holds the keys to the invites and has been automatically invalidating invites sent to me for some time now.
You can still request me for a GMail invite here.