Hi dgparent,
100,000 registered users is quite impressive, and you're right... you should be making more money with those kinds of numbers. The first thing that comes to mind is mailing your members.
The most effective format, I believe, would be to produce and deliver newsletter-style emails that have informative and short articles about whatever subject matter your members may be interested in, and along with these articles you should include promotional offers for website subscriptions or products.
With this, you'll be delivering something of value to your members (the articles) which will engage their interest and build brand trust/authority, while at the same time you'll be able to effectively promote a certain product that you can gain sales commissions from.
Good luck to you!
