By the time your campaign is Live, you can track the performance of the campaign from your dashboard.
The only thing you should do is to Go to the Campaigns Overview and select the Campaign you want to view the performance.
From the Statistics page, you can watch basic metrics (those metrics are displayed on real time) about the Campaign.
The metric Impressions reflects to how many customers this campaign has been displayed.
The metric Actions presents how many customers actually pressed on the button of the campaign.
Engagement Rate: It is the dividend of Actions with Impressions. The highest is the rate the better the campaign goes!