A traffic exchange with many members and few advertisers will not be successful. Conversely, a traffic exchange with many advertisers and few members surfing will suffer as well.
How does shareadspace make it's money?
Shareadspace uses a network of outside affiliate programs based on banners, pop-ups, and text links. These bring extra money into the system, and are what enables shareadspace to pay such a generous amount for each page view.
The shareadspace owner constantly monitors affiliate income, surfing patterns, payment rates, and all other factors that affect the financial stability of the program. And makes whatever changes necessary to ensure the books balance. This ensures the long term survival of the site, and so guarantees that members always get paid.