Here are the three I’ve found to be the most important to watch:
1. Per-Post Engagement
The Post engagement metric tells you the number of people who have interacted with (Liked, Commented or Shared) one of your Posts. It also identifies Post Clicks, but your CTR (see below) covers this variable far more effectively.
Why you Should Watch it:
Having many people see your content (Reach) is largely meaningless if they don’t interact with it. Engagement is the only measurable sign of your audience responding.
In order to get the most out of your Engagement metric you have to compare it to the total Reach of the Post:

This number gives you a way to compare Posts across the board. Without it you won’t know if good Engagement on a specific Post is due to the quality of your content, or if it was simply seen by more people.
2. People Engaged
The Engagement Demographic graph tells you the age and gender of the Facebook users who engaged with your Posts.
Why you Should Watch it:
The People Engaged metric is vital to ensure your content is reaching your target audience.
3. Post Click-Through-Rates (CTR)
CTR is the number of Facebook users who clicked on your Post or Ad (not including Likes, Shares or Comments)While it’s important to know the Engagement of your Posts, your bottom line is knowing how many people were interested in your content enough to act on your Calls-to-Action.
View the rest of the post here: The Science of Keeping your Facebook Fans Engaged

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