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