Ning

Ning allows you to create your own social networking space to collaborate with friends, colleagues, students and others in a user friendly way. However it does require some development as has been stated by Webmonkey "The Ning gnomes provide a number of custom PHP classes, both for interacting with the database and for quickly adding sprinkles of Ajax, web services, and other treats to an application. There's a lot there, and plumbing all of Ning's continually growing possibilities is a big job..." They give an example of a bookmark manager with the code.