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Scribd offers a lot of cool social publishing features for the documents that are published, and for collections that users can make, and that can be shared.

The service is offered free of charge, yet sometimes annoying and agressive adds disturb the reading experience

Description

Scribd is the largest social publishing and reading site in the world. It makes it incredibly simple for anyone to share and discover informative, entertaining and original written content on the web and mobile devices.
Scribd's vision is to liberate the written word, to connect people and organizations with the information and ideas that matter most to them.

Source: http://www.scribd.com/about

Media coverage: Time Special - Tech Pioneers: 10 Start-ups that will change your life (Sept. 2010)

Availability

Available globally with some extra services (selling of documents) restricted to the North American market.

Blueprint

Scribd offers social publishing services that are part of a global to local social publishing infrastructure, and could become (an essential) part of a lifelong learning (social reading) experience for all. These services have network effects and monopoly risks (in a niche). Some may argue that for this reason the services should/could be offered in a public-private partnership arrangements, or that some measures are desirable to prevent commercial providers exploiting their monopoly position.