Facebook engagement measures how fans and Facebook users interact with brand posts and pages.
There is different forms of Facebook engagement metrics. Some are centered on posts (see Facebook post engagement rate), some on brand pages and others are overall.
Facebook engagement may be measured by advertisers, agencies or directly by Facebook analytic dashboards for Facebook PTAT.
Facebook engagement measurement is complementary of the number of fans for evaluating Facebook presence effectiveness. Many fans with low engagement level are low value for the brand.
Facebook engagement measurement is also used for identifying best post practices (time, forms and contents of posts).