The Russian Internet Research Agency's (IRA) online interference campaign in the 2016 U.S. presidential election represents a turning point in the trajectory of democratic elections in the digital age. What can we learn about how the IRA engages U.S. audiences, ahead of the 2020 U.S. presidential election? We provide the first in-depth analysis of the relationships between IRA content characteristics and user engagement on Facebook and Instagram around the 2016 election. We find that content targeting right-wing and non-Black marginalised groups had the strongest positive association with engagement on both Facebook and Instagram, in contrast to findings from the IRA campaign on Twitter and to some previous commentary in the media. Higher engagement was associated with posting later in the 2015-2017 period and using less text on both platforms, using negative wording and not including links on Facebook, and using fewer hashtags on Instagram. The sub-audiences and sub-issues associated with most engagement differed across the platforms.