My Take
Scrible kind of does it all. It's not one of those 'fun' tools. It's also not a learning tool per se, but it sure makes learning the skills and steps necessary to do proper research a LOT easier. Research steps are always something of a chore in students' minds, but Scrible is useful in that it makes those steps easier and facilitates learning. The potential for collaboration is a big bonus.
Tools like diigo do a lot of the same things, but I do like how Scrible puts things together. Scrible also has a browser extension if you're interested and integrates with Google classroom.
How I Use It
I have created a class on Scrible and then added articles/stories/research resources to it. Then, after inviting students to the class, they can access those resources themselves for class projects. As we work in a unit or on a project, I ask students to add additional resources to the class as well - building a library for everyone to use. Students can add documents (pdfs, word docs, etc) and webpages. They can also create a research project or paper within Scrible if they link to their Google Drive account.
I have also had students use Scrible to create bibliographies to accompany their work. It's an easy way to build a bibliography as you go.