Continuing the discussion from Google+ Hangouts on Air Experts?:
Splitting this into a new topic.
I think this is an interesting question, @randulo. I tend to see the differences between forums and social media as the unit of discussion and the audience.
Forums are much better suited for richer, longer form discussions. If you look at a typical forum thread it will include a number of participants, often incorporates rich media (e.g. videos, pictures, polls, and more), and has tools better suited for responding and replying to people. Social media great for discussion, but the unit of conversation is typically smaller and often replies to that specific post. As such, I feel forums are better for longer more involved discussions and social is better for smaller more informal discussions.
In terms of the audience, I have always found social media audiences to be a little less sticky and people are instead pulled into the content when it stands out. Forums are more of a clubhouse where people join up and become invested in the community. Thus, while forums may be smaller, I suspect the quality of the content is more in-depth and longer-running.