I think it's worth noting that in that regard, Google Plus, with its 'circles' grouping mechanism, is very good at respecting one's privacy to whatever degree that they wish and are willing to put forth the effort to implement.
Yegge infamously posted a rant publicly on G+ without intending to. I don't think I'd bet my life on circles, especially after watching how Facebook put in similar features and then capriciously violated the intent of many users.
