Web forums ebb and flow.
I think that casual visitors - the ones who in the early days just join to ask the odd question, find what they want in the forum has been discussed already so read the relevant thread after finding it via Google. I still do this a lot - just dive into years-old discussions and read them. Fascinating stuff.
People have less time these days as well - economic times are tough, less time for people to think about things such as personal health, plus a general trend of people leaving or not joining formal social groups.
I suspect Facebook has also taken the people who would otherwise be using the forum for a mix of casual and weight-related social interaction. Facebook has perfected the drug of instant personal gratification and narcisism. Although I haven't seen any evidence, I am sure there are plenty of weight-losers who regularly post their weight-loss progress on their FaceBook page, with its promise of an instant flurry of "Likes" and "Woo Hoo, You Go Girl!" messages rather than the relatively slow responses of fellow travellers.
And some of us regulars have lessened in the number of message posts due to distractions outside the forum or have reached our goals and need the forum less, or have given up.
However, I suspect the people who now use and interact with the forum (as opposed to lurking in the background reading the articles and messages) are those dedicated souls who see the need for support from people who have gone through, or are going through, the same struggles as they are.