I created one with ning and I am also active in two ning networks. In Germany also mixxt.de is popular as platform for Barcamps - Sebastian Küpers
I don't think CMS vendors can do nothing. The lines between corporate generated editorial content and community generated content are blurring. It's about more than just wrapping social widgets in a webpage. Community generated content needs to be another core content type in the CMS, with full access to all of the typical CMS functions for managing persuasive site experiences. - Tom Wentworth
