There are many metrics that can help you assess you community’s strength:
- size (number of members)
- number of boards (or something similar, depending on whether your community is using boards or not)
- number of posts
- number of comments
- clicks per unique user
- posts per unique user
- and more
Basically, the most important metric nowadays is engagement. Engagement itself describes how often and how intense your users participate in your community. In simple terms: How often they return, how long they stay and how much they interact/contribute. To add to that, you can (and should) also cross-analyze data to find out how intense overall community engangement is vs. per-user engagement.
The ideal solution here is always a tailored one. For example, if you want to just measure the quota of engagement of your Facebook page fans, you can compare the number of active (participating) fans vs. the number of passive (not participating) fans.
See the answer on Quora: