Yes they are unavailable at the moment, I recieved this email from them not too long ago:
Thank you for your continued support. Currently, Hippopost is
temporarily unavailable while we pause our beta program to
prepare for a full commercial rollout. We are excited as we
update Hippopost to make it better and more exciting for you - and
the hundreds of thousands of other folks that have been using
Hippopost to send FREE printed postcards!
Regards
The Hippopost Team
P.S. Follow us on Twitter at hippopost (hippopost) on Twitter to
be one of the first to know when our new, enhanced service resumes.