For those concerned about the privacy issues raised by the new LJ facility to cross-post comments on someone's locked LJ post to Twitter and/or Facebook, please go and do this poll (which it seems LJ will take notice of).
To be honest, I'm not too concerned personally, because as a teacher I'm very careful what I post, even under friendslock. As it happens, my family already know my LJ name as do some RL friends. (It's more or less the same as my Flickr name.)
I also trust the people on my friends list and anyway I have different levels of filter for anything in the least bit sensitive. However, I can see this being an issue for many people, so while the ability to cross-post one's own posts makes sense (for those who like to do that kind of thing), being able to cross-post a comment to someone else's post makes no sense at all. I can also see the danger that people who would never intentionally betray a confidence might get into the habit of cross-posting everything and tick the box without thinking.
Another issue that no one has addressed is, will the comment make sense out of context? Facebook and Twitter and full of random rubbish as it is[*], comments to Lj posts -- which are likely to mean nothing if you haven't read the original post -- are just going to increase the noise to signal ratio even more.
[*] At the risk of being thought a middle-aged curmudgeon, the re-tweet feature on Twitter really annoys me. You can end up seeing the same thing re-tweeted by several friends. It also means I end up reading stuff I'm not interested in and has led to me dropping people I would otherwise like to follow (for their original content) but they post so many re-tweets, I get irritated.
And as to Facebook... I'm really not interested in whether someone has had a new calf born on their farm and I don't want to join their gang or whatever else these posts ask for. I wish those game applications only showed up to people who are also signed up to the game and were invisible to other readers. As far as I'm concerned, Facebook is just a way of having the contact details of people I only know online would lose touch with if something happened to LJ.