Link clicks
In Facebook Insights, you can see overall clicks on your posts, but you can also break it down further to see specific link clicks. Link clicks are the most important because they show the most relevant form of engagement. This tells you your audience is interested enough to learn more, and if they’re on your own site, you can track their journey from there.
Reach versus impressions
Reach is the number of people who see a post, while impressions are the number of times the post was seen. For example, if a post was shown to the same person twice, it would count as one reach and two impressions. If your campaign goal is to get your post in front of a specific number of people or get the word out to as many people as possible, then reach would be the appropriate metric to look at. If your goal is to have your post shown a specific number of times to your audience to increase brand awareness, then impressions would be a suitable metric to look at. It all depends on what your goal is.
Platform engagement
Platform engagement means the likes, retweets, shares, comments, follows, etc., and while probably slightly overrated in terms of its importance. They are helpful for in terms of interesting content and helping the algorithms running behind social networks to rank your content higher on timelines. Though it’s less reliable than link clicks, it can help increase reach and impressions when people interact with your content as the bulk of networks will now share liking activity with your friends/followers.