Our friends ate Keystone have just published a remarkable study of social market places that is available at: www. keystoneaccountability.org/node/159.
It tracks and examines the growth of online philanthropic marketplaces and their potential to create an informational basis for results-oriented giving and encouraging greater accountability. The study finds that while there is great potential and the marketplaces are doing remarkable work, there is still a need for clear criteria to determine effectiveness and impact.
It's a long read but worth reading through during train rides.