It is 2012, and i am surprised these "marketplaces" still exist in the same format, instead of a thoughts.username.com, books. username.com, tutorials.username.com etc, and a totally unique discovery algorithm powered by past analytics (a google scale algo version of stumbleupon).

Device lock-ins do not help either. just adds to the we have a marketplace hence users give us x percentage model.twitter, for example shd have been a protocol and not a private firm.