Say there's a startup that is going to revolutionize date keeping and events and scheduling and all that, for the sake of aping a common naming scheme, call them Calendr[1]. Only drawback is that their security is an afterthought, but they're not promoting that.
So a Facebook user that is friends with you on Facebook says to Calendr, "scan my contacts and generate a calendar that already has my contacts' birthdays and any events they've created on it (one would assume this list would include anything that is shared at the Friends Only and Public tiers) for me."
Three weeks later, Calendr is hacked and all of their data is accessible. A Have I Been Pwned-style service will let you read through the data and sure enough: fixermark's super secret event was now publicly viewed as part of this data set. You do not have an account with Calendr and you haven't even heard of it before.
How would you, as a Facebook user, prevent this from happening beyond not creating the event in Facebook? How would Facebook prevent this beyond not providing the data to the third party?
[1] edit: oh geez, there is a Calendr. This has nothing to do with the real Calendr (this is fictitious Calendr).