"... they are ... people I know on social media ..."
Depending how you configure programs like Facebook, Instagram, Twitter, Dropbox, Skype, third party email apps etc. they often have a setting that allows them to access, use, and even update your Contacts. Typically they will initially request your permission to do this, which I ALWAYS DENY, but sometime in the past you may have allowed this, in which case you need to track it down and change it. This could have happened from a web site (if you access Facebook that way, for instance), or from scores of apps on a cell phone. Even some of my Garmin devices for sports/exercise that connect online try to do this. Contacts does not do this on its own, but if you allow access to your Contacts to various apps or social media programs, they will do this.
On your Mac you can start with Security and Privacy Preference Pane, select Privacy, and see what you have allowed on your Mac for access to Contacts. However this could also be coming from your phone as well, which requires looking into the phone device settings for each individual app.