I'm not sure it's as much of a contradiction as you think it is. Noting "It manifests itself in the declining visit and posting frequency on Facebook across man

I'm not sure it's as much of a contradiction as you think it is. Noting "It manifests itself in the declining visit and posting frequency on Facebook across many cohorts", I find it entirely plausible that feeding the trolls yielded short term lift in target metrics at the expense of (much harder to measure and correctly attribute) long-term attrition of non-troll users. That hypothesis certainly fits the popular perception that FB is dying/dead. It somewhat fits my personal experience: I fairly aggressively pruned my news feed of any political content, which seems to have kept away the trolling, but the personal content that I wanted has dried up and my feed is now mostly ads and generic "recommended" clickbait. Why bother visiting?