How so?

Alice tells me she knows Bob and Charlie. I mention Charlie to Bob - completely normal human thing to do. Maybe I think they're in the same circle but I've forgotten exactly why or how I found that out - completely normal human memory behaviour. Where's the invasion of privacy?

In both cases.

Yours is understandable and forgivable. Charlie's privacy may have been violated, but it was a simple mistake.

But suppose you say to Alice, "Tell me all the people you know. I'll tell you which ones I know too." If you then write down that list and systematically start telling them about each other and that they have Alice as a mutual friend, then you're a privacy-invading jerk, and Alice was wrong to trust you.