Social Reading Platform for Math or LaTeX texts Social reading is considered to be one of the big trends that could be catalysing learning by reading. Features could include: 

 Highlighting or annotating paragraphs or single steps in a proof for yourself (ok, this is not yet social reading), e.g. in a proof you could add to the sentence "Therefore we have ..." of the original work a more detailled explanation, thus "zooming in". 
 Publishing those of your own annotations you consider helpful for others 
 Reading all or a selection of the other reader's published annotations via a "show helpful annotations" button
 Thumbing up other's annotations. 
 Asking questions to specific paragraphs or steps in a proof. 
 Answering questions asked by others and thumbing up questions and answers.
 The original text would always keep the same, only being annotated

Thus all the insights or questions you have while reading a paper or a textbook could be shared with others and learning could be far more efficient. Authors of textbooks could take into account the annotations/questions/answers, thus optimizing their text in future editions (or developing different versions or an annotated version). 
Questions: a) Is there any tool that can provide these features, e.g. LaTeX-tools or pdf-tools? b) Would the Math overflow be a good starting point to build such a platform? c) Does anybody know of such a platform that is already in place?

(Edit: Changed to community wiki) 
 A: Your "social reading platform" looks like what HTML and the WWW=World-Wide-Web was supposed to be when Tim Berners-Lee first set up a web-server and web-browser platform applications at CERN on a NeXT machine using Objective-C (I think he programmed it in objective C).  Researchers were supposed to have their web pages listing and highligthing their research with hyperlinks pointing to the publications and datasets.  If you look at the majority of academic webpages, the pulication and research interests are listed in that way.  It's just that the majority of the internet world has gone into walled gardens such as the social media pages, with the cost of entry usually being the loss of any privacy or control over what can be done with your user-provided content.  Look at the issues discussed on the meta website here at mathoverflow about why there hasn't been an upgrade to the StackExchange 2.0 software.
Wiki pages (not just that encyclopedic site that everyone uses, but a wiki page and wiki server which you can set up for yourself) allow for multiple users to modify a text using html or internal markup language.   The requirement that $\LaTeX$ be usable in the markup language may require the use of MathML, or MathJax, or the jsMath package.
I think the correct answer is most likely an internal wiki server, with password-accessed accounts for modifying the wiki-pages.  The problem is going to lie in placing a full copy of possibly copy-righted material, particularly in the case of wanting to do an "annotated version" of a research paper, or of a book chapter.  If the author of a particular paper or book chapter, or the full book itself, wanted to do the experiment and set up their own wiki for the paper or book, and allow either free-for-all access or password-required gateway granted access to allow modifications and annotations, I would be very interested in taking part in that collaborative effort.
A: This is far from an answer, but if no answer is found I thought a partial answer might be useful.


*

*Since the last point (switching annotations on and off) seems to me the most difficult, Google Wave with its time slider comes to mind. Unfortunately, LaTeX support was extremely bad in my experience. But since the Apache foundation might take over the development there's at least a future.

*On the other hand Tiddlywiki has most of the features you're looking for. Above all, it is very hackable with tons of plugins (excellent LaTeX plugins like MathSVGPlugin and jsmath). Of course, its technology (javascript, single file) does not scale well but it might offer some ideas. I use it in several projects with a small number of users. There's a wonderful copy of the Tractatus as an example for its abilities.


EDIT: Dec 14, 2010
I recently remembered this questions when I came across a tool for Wordpress.


*

*http://digress.it/ a plugin for comments per paragraph.

*There's a number of LaTeX plugins, e.g. http://wordpress.org/extend/plugins/mathjax-latex/

*There's a plugin for comment rating http://wordpress.org/extend/plugins/comment-rating/
That seems to be the best base for a multi-user approach -- a couple of more plugins could turn it into a social reading platform.
