As we cannot get actual impressions from Instagram for any non-business account, the Mavrck platform estimates impressions based on published research done by Stanford University and Facebook's data science team called Quantifying the Invisible Audience in Social Networks.
The algorithm takes the follower count of the influencer, the engagements they have received, and a number of other factors into considerations, as described in the article listed above.
In Q2 2018, we made a number of adjustments to our algorithms that take into account recent changes made to the Instagram feed algorithm, as well as a larger set of training data collected from influencers with business accounts. These changes have significantly reduced the number of estimated impressions estimated for some segments of influencers.
Additionally, as part of our current Fraud Detection project, we are looking to soon make updates to account for various levels of automated and/or fake accounts that interact with the influencer. For example, if we determine that 20% of an influencer's audience is fake/automated, we would adjust the estimated impressions accordingly.