Ranking can be done in various ways.
One option, utilized by the Discourse forums, is "Trust Levels". As posted by the developers:
User trust levels are a way of...
- Sandboxing new users in your community so that they cannot accidentally (or intentionally!) hurt themselves, the forum, or other users while they are learning what to do.
- Granting experienced users more rights so that they can help maintain and moderate the community they generously contribute so much of their time to.
Advancement through these levels depends on several criteria being met:
- Visiting the forum and reading the posts
- Visiting the forum over a longer period of time
- Utilizing board features (liking posts, replying to others)
- Not receiving spam flags from users
Criteria for advancement increases and is designed to reward those users that are engaged and contributing to your community. While it does still depend on post count, it is not exclusive to post count. In fact, that particular measure is relatively easy to meet in an active community. It's the participation criteria that is important to gaining trust.
Other options include
- Reputation based promotions. This type of system depends on up votes and down votes and encourages community participation in both the posting of content and in letting others know if the content is "good". Several forum creators use this type of system, either built in or through plugins.
- Time based promotions. Users that have been registered for a certain amount of time are automatically promoted to the next level. This method rewards your earliest users, but also doesn't encourage participation. Instead, it just asks people to register and then they can freely abandon your community.
- Combination of reputation/time/post count. This combination allows you to reward your earliest, participating members. However, if not tuned appropriately, the time based aspect could punish the newer contributing members. That is something to be aware of when setting up this type of system. On a related note, MyBB has this type of system built in.
- Contributions, unrelated to post count. Depending on the type of community you have, user contributions can be rewarded through promotions. As an example, on an artistic forum, users that contribute new art work or musical compositions could be rewarded over users that provide feedback. Without those contributions in the first place, there would be nothing to give feedback on.