There are some nontrivial UI concerns in trying to merge non-public commenters with public posts, I'll grant you that. However, it still doesn't change the fact that the tool doesn't match up to the needs of a pretty broad demographic (no pun intended).
If you make a post and I comment on it, my comment is at the mercy of your privacy settings. Facebook and G+ are both explicit about that.
Maybe I'm missing something? I'm not trying to be obtuse, I promise.
Because Facebook hates Google, you're far less likely to see public posts turn up in Google results.
Google, on the other hand, created their network for the sole purpose of bolstering their search results. This leads to a much more adversarial relationship with the service, where you have to constantly fight to keep yourself out of the spotlight.
So, while the FB comparisons may be appropriate to conversation here, they do not -- in my view -- imply a weakness in her argument. She left FB outside the scope of what she was saying and whom she was addressing.
P.S. Upon rereading, I do note this bit:
particularly when there are other social networking alternatives with simpler privacy settings.
which perhaps weakens my original argument in this comment somewhat.