1) Active free users – adding value by doing something, like creating content
2) Passive free users – adding value by just using the service, like attracting others on a dating site
Users that neither do something useful nor attracting others should be forced out (?) or forced into the paying group (maybe by offering a fix length period).
This will be another way to look at ‘try-before-you-forced-out’ model, where one can prove her usefulness to the service