Maybe some of those single-post profiles are still lurking and just read, but don't participate? I'm sure many of them have stopped visiting though.
I know when I first registered for FitDay in 2006, they didn't have forums as a feature. I was pleasantly surprised when I returned this year to find discussion boards, as support is a huge part of succeeding in a lifestyle change.
I have visited some of the other free sites, like SparkPeople, CalorieCount, etc. and used to belong to a pay site (DietWatch), but still find FitDay to be best suited to my personal needs.
It's a clean, simple layout and navigation system, and just doesn't seem as cluttered up with ads and foo-foo features.
The only thing I wish FD did better was have active moderators or other presence on the site, because right now it seems like they are invisible. I would also love it if they had a chat room function. There are times when I log on here to post and I really would love to just talk to somebody in real-time.