Matt Rice is ranting about The Flash Message Board Life Cycle over at SwfNews. I think he has formulated the life cycle of a successful message board pretty good, be it a Flash message board or a message board for anything else. I have seen this in news groups, web boards and to some extent in mailing lists.
The problem is in the numbers, and the solution? Probably to make a taxation system of some kind. Personally I am of the opinion that users that just consume should have to pay for their consumption, whilst users that contribute to a message board should be paid for it. It does not necessarily have to be with money, but there should be a cost/benefit to using/providing for a message board. A taxation system would probably keep in check the cricket-pest like conditions that message board experience when Phase 4 (see Matts message board system) kicks in.