The button appears under everything, at a price low enough to look harmless. Most of the money spent on it buys reach for posts that had already been judged.
The rule that survives
Only promote what is already working organically. Paying is amplification: it makes more people see the post, not more people like it. A post that fell flat with people who chose to follow you will fall flat with strangers, more expensively.
What to look for first
- Saves and shares above your usual — people found it worth keeping
- A high share of reach from recommendations — the platform already likes it
- Follows from that post — it recruits, which is the thing worth buying more of
What not to promote
Anything time-limited that has already nearly expired, anything you have not read again since publishing, and anything whose main measure is likes. Likes are the cheapest signal and the easiest to buy accidentally.
Set the amount before you look at the result, and treat it as spent. The failure mode is not overspending on one post — it is topping up a mediocre one because stopping feels like admitting it did not work.