The term comes from the startup world. Eric Ries, author of The Lean Startup, contrasted vanity metrics with actionable ones: numbers that show cause and effect and can guide a decision. The same metric can be either. Followers are a vanity metric when they are the goal, and a useful one when you track which content brings in the right people.
A quick test is to ask what you would do differently if the number doubled or halved. If the answer is nothing, it is decoration. Useful reports pair reach and engagement with the outcomes they are meant to drive, such as replies, clicks, enquiries and sales, and look at trends over time rather than single spikes.
