MAU afaik is the easier number to find closer to actual engagement.
But I think engagement could be measured by (comments + votes + downvotes + boosts if available) / (number of posts).
However, size of a given instance, specially for smaller ones, may cause odd fluctuations, so standard deviation or a threshold of small/medium/large instances may be needed when ranking.
Also an instance that had a massive initial engagement metric, or an abysmal one, would have its overall score affected for quite a while, affecting the interpretation of others about its metrics. So to mitigate, maybe taking the numbers by time period may be better, e.g. one ranking for the past week, one for the month, one for the semester, one for the year, and one for all time.