  Ensuring widespread public exposure to best-science guidance is crucial in a
crisis, e.g. Covid-19, climate change. Mapping the emitter-receiver dynamics of
Covid-19 guidance among 87 million Facebook users, we uncover a multi-sided
battle over exposure that gets lost well before the pandemic's official
announcement. By the time Covid-19 vaccines emerge, the mainstream majority --
including many parenting communities -- have moved even closer to more extreme
communities. The hidden heterogeneity explains why Facebook's own promotion of
best-science guidance also missed key audience segments. A simple mathematical
model reproduces these exposure dynamics at the system level. Our findings can
be used to tailor guidance at scale while accounting for individual diversity,
and to predict tipping point behavior and system-level responses to
interventions.
