The following is a list of key metrics that organizations choose from to track the effectiveness of their communities:

Unique Visitors
New Member Registrations
Page Views
Retention / Attrition
Member Loyalty
Member Satisfaction
Most Active Members
Top Searches
Message Posts
Conversion
Advertising Performance
Influencer / Evangelist
Identification
Member Lifecycle
First Time Contributors
Content Rating
Ratio: Unregistered to Registered Visitors
Ratio: Page Views Per Post
Reputation Changes
Ratio: Posts Per Thread
Content Tagging
Comments per Blog Post
Ratio: Searches Per Post
Podcasts & Video (linked to / Uploaded)
Member Blog Posts
Size of Networks / Buddy lists
Quality of content and exchange
Tracking the brand through the “Community ecosystem
Impact of the community on revenue
Mobile interactions with the community

Source: Forum One Communication