Such a class of information doesn't exist. I tell them, and then I expect that anyone else who might be interested will find out. The information is no longer secure.
The key to operational security isn't that no one ever finds out; it's that no one finds out until it's too late for them to stop you. Perimeter security is a war of attrition: you will take losses and you don't get to pick them.
I have confidences I've given to friends. I don't expect them to share those secrets with others, but I'm also aware that they very well might, even accidentally. (Or the sharing got MITM'd.) Thus, instead of using shared confidences as magical shibboleths to friendship, I use them as the first step on increasing my comfort level with sharing them publicly. For instance, I was extremely ashamed of the fact that I watched porn when I was a teenager; now I can say it freely on a public forum with my name attached. There are a dozen steps of infosec stepdown that occurred as I grew up between then and now.
