The first site, the one with the free membership and low-cost training is based on a just-in-time model. Members simply pick what they want and pay their $5 or $10 for the training they want. As part of the membership a number of no-cost classes are provided. This alone keeps people on the list. The model appears to be sound. Getting traffic to the site is how they add to the list and offering low-cost products and services is where they boost the income. A free membership site can gain a huge following as long as there is constant value being offered. A focus on relevant content plus timely bonus offerings is key.
The second site also has a free membership. Members are from a narrow niche and the offerings are catered to that niche. The site is a go-to location for finding the latest books and other products, sometimes at a discount. The site does not make money from the sale of the books, it makes money from advertisers to the list in the form of specials. Advertising is restricted to only items that are well-tailored to the audience. Since the site is so targeted, Members are gathered from niche groups, social media advertising, and other locations where the target audience gathers. Is it successful? Absolutely!