I recommend reading the comments for both facebook links and the dances with fat link.
The overview is that knee problems have many causes, and careful examination and physical therapy are the best starting points for people of all weights. Evidence that weight doesn't necessarily matter is that there are people who have the same knee problems at a variety of weights, and people with congenital knee problems who have relatives of various weights with the same problem.
Medical neglect of fat people is a serious thing-- they are frequently told to lose weight to solve problems and not given other treatment, including problems which aren't conceivably related to weight. In many cases, fat people suffer for decades from problems which are easily solved when they get proper treatment-- or find out that the problem can no longer be solved. The worst case I know of in at the Dances with Fat link-- a fat woman whose back pains turned out to be bone cancer, and it was too late.
I'm not going to say losing weight never works-- it may have done my knees some good*-- but a lot of the time, it doesn't work, and other solutions do work.
*Hard to tell-- it might have been the qi gong.
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