Hi guys. I realize now this piece of magic was not built by Sketchfab but Stéphane Ginier. I was so "wow" that I didn't check and just posted it here. I'm a Sketchfab-enthusiastic but this was not a promotional message. My bad.
This is absolutely amazing! However, it reminds me a lot more of the early versions of - <a href="http://pixologic.com/sculptris/" rel="nofollow">http://pixologic.com/sculptris/</a> - than it does of ZBrush.
Nota bene: Sculptfab is an experiment made by Sketchfab on top of the amazing SculptGL tool developed by Stéphane Ginier, in order to make the workflow of publishing a model to Sketchfab easier :)
Since when do we congratulate companies for slapping their logo on someone else's open source project? And this after Stephane added a Sketchfab link in the original. Seriously guys, lame.