Yeah, this is the really, really important bit.
I noticed a pattern when I checked threads about how much money people make off Tapas and observed how many likes and comments different comics were getting: My comic was punching well above its weight in ad income for the sub count. I was substantially outperforming comics with more subs than me.
Because I focus on more meaningful interactions with my promotion, the readership I pick up is made up of a higher than average proportion of actual engaged readers. They didn't just click "subscribe" in the hope of getting one in return or to maybe read later, they click on my updates, give me likes, comments and ink donations. The raw data is there in my ad revenue, it's higher than other people's because my updates are getting more views.
More likes additionally means a higher place in the "Popular" rankings, which means more visibility outside the forums, and the holy grail: More visibility on the app, which is the place you really want visibility because that's where the vast majority of readers are. Likes are arguably more important than subs, and I also think they give a much clearer indication of how popular a work really is in terms of audience engagement.