Below is a screenshot of a webpage Amazon.com has for a finance book that presumably is used in colleges and universities.
There are more sections of highlighted text on the page that are not shown in the screenshot. The Public Notes section has no notes. The readers, students, could write in notes for a section of text and their notes would be easy to see on their own Notes page on the Amazon site. Notes they make public would presumably appear in this text box.
What would be interesting is if it would be possible to join a group, think class or book club, to read and share notes with. This could engage students and readers more. If half the class highlighted some text it would probably be worth talking about in class. The notes people write would enable a better discussion because reader recall would presumably be faster when text the person wrote was presented. The internal database of the mind would be faster for searching and synthesizing the thoughts and ideas of the book and passages.
Link to the page for the book screenshot above. Perhaps more passages will be highlighted by the time you visit.