Webjam looks like social start page version of Squidoo. Actually the powerful Zimbio is a communal type of Squidoo, where people can join/contribute to a topic. Zimbio has lots of modules for each topic (link lists, blog, forum, etc…), and you get a dashboard view of all your modules on a startpage.
So I guess Webjam seems similar, since it to has a blog widget and a bookmarks widget, but Webjam in comparison is just the startpage component.
I really like the feel of Webjam, it is easier enough to use, and fun to create a topic start page.
I guess the difference with Webjam, like Ning, is that you can clone topics and make them into your own.