One of the better options for this type of platform is the Drupal Content Management System (CMS). Its abundant set of features makes it the ideal platform. I will highlight a few features.
Organic Groups - This features helps to organize content around groups or sub groups.
Taxonomy - Categorizing content around tags, taxonomy terms helps users find information easily but it also creates a richer user experience
Faceted Search - Building content around facets helps users find information very easily using a combination of filters, tags, terms
Messaging - Instant messaging between users or groups
Interactive maps, Views(Query Builder),
While working on a World Bank project, understandrisk.org we incorporated some of these features.
The other powerful aspect about Drupal is that it has matured beyond simply being a CMS and has evolved into a framework which means that you can build anything with it, interface with external applications based on your unique requirements.
Technical Lead - Blue Node Media