I am looking for free online forums where you can set up group communication, for informal exchange and learning with the ability for parallel threads. I think it is good for groups that don't want to invest heavily in customized forums in the initial group stages, or are not sponsored by an organisation. The fact that the forum is free and easy to set up and delete can make the online experience for participants more experimental. You avoid the fact that people feel that they now have to communicate online because their organisation built such a forum and invested this amount in building and hosting it.
For email-based discussion forums there is yahoo groups en google groups, but there you have to follow all messages or nothing (not completely true, you can follow it by RSS too, but you can't ignore certain threads)...
I am quite a big fan of NING, but I think it might seem chaotic for some. The advantage is the flexibility (you can insert element, or leave out elements) and the option to insert RSS feeds.
So far I've come across the following alternative: collectivex. Josien pointed to Bryght and Barnraiser. Oscar mentioned Goingon, Mugshot and Peopleaggregator. In my delicious I further have Razoo, Grou.ps and Onlinegroups. And there is the alternative of starting a group on a site where people are already active, eg. facebook.
On what basis should you choose one or the other? Is there a way to avoid that the preference of the initiator is the only factor guiding the choose of a certain platform?