A project of the Elgg Foundation, Elgg is a free open source social networking engine licensed to users under the GNU Public License v2. The REST Elgg API allows developers to integrate Elgg and offer new apps for it. To give a sense of its size, 1,646 plugins have been created so far by the user community, with nearly 3 million downloads. Examples of plugins include blogs, forums, social bookmarks.
Users include Harvard University Extension, NASA, Wiley publications, the World Bank, and the Canadian Government, as a representative sample. What ties them together is each needs its own social network, either internally, or to relate to its constituents.
Cash Costello, a core developer of Elgg, wrote the book on it, Elgg 1.8: Social Networking, in early 2012, and was praised by reviewers on Amazon,
"Elgg Official Documentation is not well organized for beginners so it is quite difficult to be an early plug-in developer. Elgg 1.8 Social Networking covers exactly this failure and let you learn all Elgg basics in a sequential and very clear and well organized order."
With a foreword by Elg co-founder Dave Tosh, it delves into plugins, and shows how you can develop and share your own. As the Elgg site makes clear, plugins are critical to Ellf's development.