Engagement is often reported as a single total, with likes, comments and shares added together. Ranking systems do not treat them as equivalent, and neither should the reporting.
Effort is what makes an action informative
A like costs almost nothing. It can be given while scrolling, without reading, and is frequently given out of habit or politeness.
A share requires a decision to associate yourself with the content in front of other people, which is a materially higher price.
Ranking systems weight signals by how costly they are to produce, because expensive actions are harder to fake and correlate better with genuine interest.
Private sending is the strongest signal available
Sending something directly to a specific person is the most deliberate action a feed offers. It requires choosing the content, choosing the recipient and accepting the social consequence.
Platforms weight these heavily, and several report them separately precisely because they predict downstream performance better than public metrics.
They are also the least gameable, since there is no visible count to inflate and no reciprocal exchange to arrange.
What earns a send is not what earns a like
Content is sent when it is useful to a specific person, when it is funny enough to be worth someone else's time, or when it settles an argument.
Content is liked when it is agreeable. Those are different properties and they are produced by different creative decisions.
Optimising toward likes therefore drives output toward the inoffensive, which is exactly the material that never gets forwarded.
Saves indicate intent rather than approval
Saving a post is a private commitment to return to it, usually because it contains information the person expects to need.
For commercial content that signal often predicts purchase better than any public reaction, since saving a product is close to a shortlist.
It is invisible to competitors and to casual observers, which is one reason accounts with modest visible engagement sometimes outperform louder ones.
Reporting should separate the signals
A combined engagement rate can rise while sends and saves fall, which means the content is becoming more agreeable and less useful.
Tracking the ratio of shares and saves to likes gives an early read on that drift, well before reach declines.
It also changes creative briefs in a specific direction, because asking what would make someone send this to a friend produces different work from asking what people will like.