A KPI follows from a goal, not the other way round. A gym that wants new members watches trial sign-ups from posts. A brand that wants to be remembered watches reach and saves.
Keep a KPI to one or two numbers per goal and check them over the same period every time. Likes are rarely a KPI on their own, as they say little about what the business needs.
Example
A restaurant's goal for a month is more weekday bookings. Its KPI is the number of bookings made through the link in its posts.