Rather than looking at the total payouts for promoted posts, we should be looking at the total number of posts that used the promotion feature, how many of those posts covered the promotion costs after payout, and whether or not the payout was higher than their average payout from prior posts.
That's a lot of data to collect, but that's really the only way to get a more accurate picture of the utility of the feature. Looking at total earnings without acknowledging previous success of the users doesn't give an accurate measure of the potential benefit of the feature.
Like I mentioned in my first comment - if someone averages $500 per post, then promotes a post for $100 and earns another $500 payout, we can say that they likely lost $100 on the promotion. It didn't actually improve earnings.
Anyway - I'm glad to see your stats and it'll be interesting to continue watching how this evolves. Maybe a program can be created to calculate what I mentioned above?