I guess it depends on the type of knowledge you're looking for. If you want to learn about the experiences of others and how they handle their problems forums are a great place to gain knowledge. If you're working on your doctorate in philosophy, there are better places.
There is much to be gained in sharing your experiences and what you've learned from them with others. At the very least you learn that you're not alone, or much different or out in left field. The human experience is one we can all share and learn from and forums are a great place to do that.
p.s. You also learn about great sites that do offer more in the way of 'knowledge'.
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Philosophy and Personal Development