When Adam Mosseri took over Meta-owned Instagram as CEO in 2018, the app was experiencing what he’d later call “concerning” drops and plateaus in user engagement, thanks partly to fierce competition from a new app: TikTok. Instagram estimated in 2019 that 23 percent of the decline in time spent on Instagram in the US was due to TikTok. Bytedance’s video app kept expanding through the onset of the covid-19 pandemic. “We can’t explain it all, but what’s clear at this point is…