I bought an iPhone from Facebook Marketplace and it's activation locked, how can I contact the original owner?

Apple will not help you. We cannot help you. You've most likely been scammed, which happens every day. Try to get a refund, but you likely won't find the person who sold you a most likely stolen iPhone. And if you can't get a refund, recycle the phone as it is positively useless to you.

Read this --> https://discussions.apple.com/docs/DOC-250001038

You dont.

Activation lock is a security protocol that can't be bypassed. Only the person it is locked to can unlock it.

The person who sold it to you knew it was stolen, they are long gone now. Contact Facebook Marketplace and see if they will reimburse you.

The All Too Common SAD Reality of Buying a used phone - Apple Community<https://discussions.apple.com/docs/DOC-250001038>

Nothing you can do about it other than attempting a refund. It is not unusual for a thief to post stolen iPhone's on FB Marketplace and in that case you are out the money. That is the risk you take when buying a used device from any third party.