Based on my conversations with those both successful and struggling with their social media participation and optimization, much of the troubles stem from a simple lack of understanding. There's a lot of "overlap" in the reports, which I believe is certainly one reason this data doesn't generate more attention and exploration.
Let's look at these metrics in more detail and consider how you, as a social media maven, can use the information to your benefit.
Reach: The number of people that have seen your post. The Reach metric is segmented into three seperate channels - organic, paid and viral.
Organic reach is the number of people that have seen your post in their news feed, within the ticker, or on your page. Keep in mind that metrics reported under organic reach can include people that have never liked your page.
Paid reach is the reach associated with sponsored stories or Page Post Ads. It is reported as the number of unique people that saw an ad or story that pointed to your page.
Viral reach is where things get tricky. Viral reach is the number of people that have seen your post because one of their friends interacted with it, either liking, commenting, sharing, answering a question or responding to an event. Viral reach also has to do quite a bit with the Talking About This (TAT) metric discussed below.
Engaged Users: The number of people who have clicked within your post/update.
The Engaged Users metric is reported in three forms: link clicks (clicks on links within your post/update), stories generated (a story is created when someone likes, comments, shares, answers a question or responds to an event), and other clicks, which are clicks on names, timestamp or the number of likes (the "other clicks" metrics is a good indicator of attention).
Talking About This: The number of people that have created a "story" from your page post. The TAT metric reports the number of likes, comments and shares in aggregate.