The idea is one of the best ever! I still think it needs a few minor changes though. I think it needs a private and maybe a selective private option. Also it seems to work better if you have a small tree. When you have large trees it then tries to force them into combing randomly. I think that it would be better if they could find a way to allow more options at the base of the tree and then split them out for us power users.
Pearltrees is a social network with tools and processes to collect web content then and it very easily. The social aspect of Pearltrees then allows your content to be viewed, searched and linked together by others with similar interest. It's a brilliant idea!