So here's how it could have happened.... You may have brought it online in which case almost certainly there was a FB tracking pixel fired back from the e-comm

So here's how it could have happened.... You may have brought it online in which case almost certainly there was a FB tracking pixel fired back from the e-commerce website to signal that you did. On the other hand if you bought from an offline store and swiped a loyalty card while doing so, your transaction might have been uploaded to create a custom audience. If you paid by your phone or have your loyalty app on your phone this becomes even easier - convenient offline to online matching is the only reason loyalty program apps are pushed. Finally, even if you didn't do any if these and the other person did, if you both connected to the WiFi at the party and checked FB (or better still you added the other person as a friend), the shared IP address between your devices is enough of a signal to begin connectings.