For knowledge sharing purposes chat is way too ephemeral.

Forums have an ephemerality too as threads get too cluttered or buried under new threads. When an old thread takes too long to find or filter through, new ones are created and then redundancy sucks up everyone's time.

The answer is moderation - preferably with "elected" moderators. The other key is to have a system of escalation of key knowledge to a Wiki, which again needs moderation in the form of reviews to keep the knowledge up to date.