Eventually, a user gets to the point where he’s done everything the gamification site has to offer, many times over. Even though he’s built up a large gameplay presence, he’s still thinking about leaving out of sheer boredom. What to do?
Postpone the inevitable. Don’t let the user finish everything, ever. If you have 10 levels, and it takes four weeks to progress from level 8 to 9, make it take ten weeks to get to level 10. You don’t want every collection or progression to be unattainable, but make sure there are a few that basically are.
Allow new privileges. In a traditional game, players who beat the game are often rewarded with a new mode; they get to play the whole game over again with a few rules changed. While this new version might be harder, the players are given some new powerful ability. In the same way, you can give your users new privileges — even let them have some control over the site.
Introduce extensions. Try introducing new features that are available only to elder users. If no new features are available, consider withholding features from early and mid-game users. At the very least, you should add new achievements with increased requirements that are above and beyond anything you already have.
Throughout the game cycle, gameplay rewards must be exciting (read: new), issued frequently, and meaningful to the user. The experience should not be too repetitive; otherwise, users will get bored.