  The global spread of the novel coronavirus is affected by the spread of
related misinformation -- the so-called COVID-19 Infodemic -- that makes
populations more vulnerable to the disease through resistance to mitigation
efforts. Here we analyze the prevalence and diffusion of links to
low-credibility content about the pandemic across two major social media
platforms, Twitter and Facebook. We characterize cross-platform similarities
and differences in popular sources, diffusion patterns, influencers,
coordination, and automation. Comparing the two platforms, we find divergence
among the prevalence of popular low-credibility sources and suspicious videos.
A minority of accounts and pages exert a strong influence on each platform.
These misinformation "superspreaders" are often associated with the
low-credibility sources and tend to be verified by the platforms. On both
platforms, there is evidence of coordinated sharing of Infodemic content. The
overt nature of this manipulation points to the need for societal-level
solutions in addition to mitigation strategies within the platforms. However,
we highlight limits imposed by inconsistent data-access policies on our
capability to study harmful manipulations of information ecosystems.
