How this can become public:

You are dumb enough to use the share feature.

No leaks so far. But it is of course always a possibility. As it is with almost any other web service.

How this can get accessed by others without being explicitly public to everyone:

• Law enforcement, the justice system, and intelligence agencies
• Your conversations will be used for training LLMs of at least the company whose LLM you’re using. Data deals with others are a possibility.