Damien,
Auctomatic is seaside based. The Hawthorn Center ( www.hawcenter.org) ) uses seaside. Many universities and companies uses Croquet, ask Darius Clark, John Dougan or Julian Lombardi. Rob Vens has a few more names I think; http://www.robvens.nl/lang-en/blog .
I have figures about the numbers of users worldwide I can digg up. Want me to do that? I would be very interested to hear your results, and publish them if I may.
My profound thanks for the developer images, I could not imagine living without them anymore.
Merik Voswinkel