Talking about Cancer on Facebook

If we're not trusting facebook (and I definitely count myself as NOT trusting facebook) ... why do we trust the engagement numbers they produce? Why do we think the likes and comments have meaning? Have you never posted a comment the recipient never saw? Have you never seen comments from people no one knows?

The advertising business is worse. I kept seeing ads for out of state political races last election; was that really what the advertisers wanted? Why do we trust the FB metrics there?