As far as using forums as a pure form of promotion, I agree that you'll never see a real "return" on your time as measured by the numbers. I do, however, think that forums can still provide value. I may not be able build a critical mass of users by posting different threads, but its a great way to meet and learn from potential users. In the customer development model, I would place forum use under customer discovery and validation. Company building is out of the question here.