Amplification Rate is the ratio of social shares per post to the number of overall followers.
Coined by Avinash Kaushik, author and digital marketing evangelist at Google, amplification is “the rate at which your followers take your content and share it through their networks.”
Basically, the higher your amplification rate, the more willing your followers are to associate themselves with your brand.
How to track it:
STEP 1: Add up the number of times a post was shared (e.g., retweeted, repinned, regrammed) during a reporting period.
STEP 2: Divide that number by your total number of followers and multiply by 100 to get your amplification rate percentage.
Source: https://blog.hootsuite.com/social-media-metrics/