Online and social bookshelves (also known as Social Reading) allow you to share what books you own (or the books you've read) online with others. You can add reviews to the books you've read, join online forums (fora if you are precise), and find new titles by looking at the libraries of others or using recommendation services.
Learn how to create an online bookshelf using WorldCat Lists here (1/15/2015: Neither the WSU Libraries nor the Orbis-Cascade consortium's Summit system use WorldCat Local any more, although users of Open WorldCat will still be able to see some WSU and Summit Libraries content in that system. It is FAR better to search the WSU Libraries' up-to-date Search It library catalog, however, if you want to access WSU Libraries books, articles, and more!
These applications work with ereaders to share your notes/annotations. Most ereader platforms will haev their own social reading capabilities, i.e. the Amazon Kindle lets you share notes and quotations on Facebook or Twitter, and also via your Kindle profile and page.