Depends on the users you are targeting. For me it was Computer Science students in India who are preparing for a competitive exam called GATE. So, providing previous GATE questions and answers was sufficient to attract users. I never considered it a business model as I was sure I'll get only loss from it, but it was something I wanted to do and Q2A was the perfect tool for me. 100 users was not a problem as once the site had like 100 questions and answers a lot of users came. Some of them contributed with answers too, but the site always needs some admin. And ensuring site is properly maintained and good SEO will bring lots of traffic by Google. For me >60% of traffic is from Google. If you are looking for 100 users who will contribute with quality answers, I would say its a tough target unless you concentrate on something special. On the otherside you can easily get 1000 of users who will be asking questions.
