PostEaseWhat separates the two
A vanity metric looks impressive but does not tell you what to do differently. An actionable metric changes a real decision depending on what it shows: whether to post more of something, change a format, or stop doing something entirely. Follower count is the clearest example of the former; engagement rate relative to your own baseline is closer to the latter.
Follower count in particular is worth de-emphasizing
A large following with weak engagement generally produces less business value than a smaller, engaged one. Follower count says little about whether the people counted are seeing or acting on what gets posted.
A practical filter for any metric
Before tracking a number regularly, ask what decision it would change if it moved significantly in either direction. A metric that would not change any decision either way is closer to a vanity metric, regardless of how central it feels to the dashboard it lives in.