A friend of mine in Paris who has been professionally (meaning it's her job) moderating worldwide online discussions for years, uses mailing lists (which she calls "fora" or "forums"

, instead of message boards like FB or SMF. She tells me that if you want people to participate in these dialogs the messages need to go to them via email. In other words, if someone has to log in to a specific website to read posts and post replies, they are
less likely to participate than if the posts arrive in their inbox and they can post a reply via email, as suggested elsewhere in "Wishlist and Suggestions".
I'm not suggesting that folks should dump FB and use a mail list, but I
do think that if users subscribe to a thread, their email notifications should include not only new post's
subject, but also the
message text. Perhaps this could even be an option set through each user's profile. The result would be a more flexible system appealing to a broader user base. And this
is a feature of some existing forms systems, like phpBB and the forum built in to Exponent CMS
http://www.exponentcms.org/.
Actually, in my dreams, you should also optionally be able to reply to a thread by replying to the notification email. FB would then be a complete hybrid between forum/message board and mail list.
Having had the experience of spending considerable time installing, templating and configuring SMF for an organization with over 1000 members and seeing only about twenty posts in a
whole year, for me it would be a dream come true.