People compare themselves to one another both offline and online. The specific online activities that worsen social comparison are partly understood, though much existing research relies on people recalling their own online activities post hoc and is situated in only a few countries. To better understand social comparison worldwide and the range of associated behaviors on social media, a survey of 38,000 people from 18 countries was paired with logged activity on Facebook for the prior month. People who reported more frequent social comparison spent more time on Facebook, had more friends, and saw proportionally more social content on the site. They also saw greater amounts of feedback on friends’ posts and proportionally more positivity. There was no evidence that social comparison happened more with acquaintances than close friends. One in five respondents recalled recently seeing a post that made them feel worse about themselves but reported conflicting views: half wished they hadn’t seen the post, while a third felt very happy for the poster. Design opportunities are discussed, including hiding feedback counts, filters for topics and people, and supporting meaningful interactions, so that when comparisons do occur, people are less affected by them.