There have been a bunch of these companies. We started one company in 1999, which we eventually moved to San Francisco. We built sticky notes which were stored together with a cash copy of the website on our servers. So one could collaborate around the notes in a corporate environment. Funding dried up in the dotcom crash, so we went home again. (An interesting side note to that was that the CIA wanted our stuff to build something for the presidents office, but decided against it, probably for security reasons. At least I got a trip to Washington DC out of it and meeting CIA CTOs or equivalents. Interesting times.)
Other companies built public note systems. I think I have seen another four or five since then that have tried to make a business around sticky notes in websites, but nothing seems to… stick (ahem).
People seem to want to give internal feedback to the content team on PDFs or in emails.
One of the original browsers, maybe Netscape, had comments on pages, but it got removed early on.
