I think the current system has a lot of collaborative features, but it does not actively remind or encourage users to collaborate. Maybe it would be useful to show a timeline style panel of collaborators' activities, such as the one we see in Facebook, where you can find things such as "XXX just posted a picture" and "XXX just liked a link". Similarly, the system can show things such as "XXX just searched xxxx", "XXX just annotated a page", etc.
I think the current system has a lot of collaborative features, but it does not actively remind or encourage users to collaborate. Maybe it would be useful to show a timeline style panel of collaborators' activities, such as the one we see in Facebook, where you can find things such as "XXX just posted a picture" and "XXX just liked a link". Similarly, the system can show things such as "XXX just searched xxxx", "XXX just annotated a page", etc.