I'm an analyst by trade which makes me somewhat annoying... but - did you look at views per thread? For instance I read DrBill's threads religiously but I don't post. There may be people reading the diving threads that are learning but don't feel they have anything to add.
Also - reviewing what the site is, doesn't mean it's what the site will be. The only way to change something is to be involved. Like LLDN mentioned, start some of the threads up again. If they came up on my list of new posts, I'd be out there reading them. As it is now, I only search through them when I have a specific question.
I like both aspects of SD. I like that there are a lot of experienced divers out here that can address any questions I have. (I just bought a new wetsuit based on advice I got here.) I also like the chit chat of the community.
I sometimes fall into the read but don't post category (with .11 posts per day). I do agree that the way to keep the diving threads going is to post in them.
I think SD's niche may be getting divers together, particularly ones that are new or getting back into it. Many of the new divers may be intimidated by the larger sites when asking for advice. For technical advice, a larger site or a tech specific site may be more appropriate. On the other hand, drawing from a smaller pool may make the advice more personal. I think this site benefits from having the more experienced divers around to answer the less experienced divers' questions.