Social Networking, Blog Comments, Twitter etc. are all good at what they do and their functions overlap somewhat with forums. But only forums provide a framework for structured long-form debate among interested parties. Sites like Reddit or Slashdot are half-way houses, having structured debate, but lacking the community feeling you get in forums.
There's definitely a critical mass - not just of users, but of quality content. We have far more lurkers than active posters and they convert to posting quite slowly. But they stick around. I attribute this to the fact that forums are moderated. We keep the troublemakers out, and generally maintain a certain atmosphere. As a result, the users form a community spirit and spread that atmosphere without any prompting.
I like to compare a good community forum to a local pub (analogy may not work so well outside of UK!). There's a large number of regulars, and long-serving bar staff. Blundering in through the door and making a scene will attract the attention of everyone. Taking time to suss the place out and joining in at a leisurely pace is more rewarding. There's a widespread feeling that it's "our" place, not just the landlord's. But ultimately he can ban whoever he wants.
Social networking sites don't have the same feeling of place. They're like a massive public square with huddles of people standing around. You can freely move around from huddle to huddle, but each one is much like the last. And you generally can't be bothered to put much effort into your content, as you don't feel any ownership over it. If you shout at the top of your voice, lots of people will hear you, but you won't really care for their responses.
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