The Selected Papers Network is a new effort in this direction that intends to federate content from all over the web. To post something to it, just write a Google+ post with #spnetwork and the paper's arxiv ID or DOI in the body. You can also post things directly at the site. The developers are working on interfacing with other social tools like Twitter.
You can read more about the thinking behind it here: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3264905/.
Edit: The site was shut down several years ago.