[sbuttgereit]

I expect that such forums actually exist to some extent and simply aren't very widely known or thought of as what they are. Facebook has name recognition and the advantage as being front-of-mind as a gathering place for friends and family; a place for serious and non-serious discussion. Other, more open forums, are likely to be more topical and niche in terms of appeal. In fact, it's likely that because Facebook is a gathering place first for the non-serious friend/family sort of communication, that you get any diversity of opinion and viewpoint at all.

Having said that, you are very unlikely to be able to avoid point of view constraints on what's allowed to be posted. That might be officially via administration or by the biases of the group. Even on Hacker News, certain points of view are favored over others and the up/down vote system enforces that outside of Hacker News policy or administration.