1. Users earn credits by being active on the site (posting comments, messaging friends, reviewing content, etc).
2. Credits can be used to do certain actions that are not possible without them (functionality that is desirable to users).
3. Credits can be sold to other site users for real money.
Time rich cash poor people can earn money by generating excess credits from their engagement, which can then be sold to time poor cash rich people who want to access the locked features.
The features would not be purchaseable other than via the credits bought from other users. This mechanism would create a supply/demand balance that should drive user engagement with the site.
How does the site make money? Advertising obviously, but also the site takes cut of credit to cash trades.
Some examples of activities on websites that could generate credits:
- connecting people - put two people together who both subsequently confirm friendship
- sharing content that friends "like"
- encouraging their friends to visit the site
- acting as a moderator or admin
- making comments that use positive language (might need an irony detection algorithm...)