I have been getting messages that people have added me as a "friend." What does this mean? (I have not seen any indications on either my desk top version of Duo or the IOS version that there is a social media aspect of Duo.)
For someone to add you as a "friend", they would need to click on your username and then press the green "follow" button on your profile page.
Duolingo is an educational site, so for the most part interaction with other users is mostly limited to discussions on the forum - however, by "following" someone you can see how many XP points they are earning daily and use that as motivation to keep on top of the scoreboard.