Measuring Product Market Fit
Figure out what metric represents delivering the value to your customer, and what would be the ideal frequency for that to happen.
Facebook users should visit Facebook daily, Uber users should order rides weekly/monthly, Airbnb users should book apartments annually.
Then use that metric to measure your retention(how many users hit that target metric). If retention gradually drops to zero, your product is bad, if it flattens out at 10% it's good, 50% is great.