Impressions refers to how many times your posts showed up on a timeline or newsfeed. This KPI is calculated individually across platforms.
Mentions refers to the brand keywords of your product or company that people use on their posts - this can be tracked using specific tools.
Followers are social media users who have opted to receive your content on their timeline.
Like emails, Click-through-rates for social media refers to the number of followers who click a link on your social post that takes them to your website or blog.