OK, so ladypeyton and I did a little experiment, with me commenting to her f-locked post (posted and locked just for this test) and x-posting it over to Facebook, then having someone who didn't know her click the link to check it out. Here's what we got:
1.) My comment to her (F-Locked) post was, "Testing now, to Facebook..."
2.) What came up on Facebook was the following status update (screenshot):
3.) So, it copied over the full text of my comment. When I clicked on that linked text, it took me back to ladypeyton's locked LJ post... but then, I was able to read it initially, so that didn't tell me if someone who WASN'T included on the LJ "Friends filter" could read it. So I asked if someone who WASN'T a friend to ladypeyton would click the link.
4.) My friend Nikki did so (she doesn't know ladypeyton on LJ, FB, or IRL), and got an "access denied" message.
IN CONCLUSION, here is what seems to happen...
If someone crossposts a comment that they made to one of your locked LiveJournal entries at Facebook or Twitter, then the TEXT OF THEIR COMMENT will post over to the status update, but otherwise, none of your original post will be seen. And, while there will be a link back to your LJ post, only those who are on the filter used for that post will be able to follow the link; anyone else will get an "access denied" message.
This does NOT alleviate most of my concerns, however. First of all, someone may have information in that comment that you would have preferred not be available to the public (for example, what if the comment was "I'm so sorry it didn't work out for you and Bob" if you didn't want anyone outside of your filter to KNOW about Bob?). Secondly, this tells anyone NOT on that filter that you posted the comment in the first place, and you may not wanted that information out there. (Yes, I know that someone can easily find out if you posted a screened post, but they'd have to actually WORK at doing so. Here, they just get handed the information!) Also, what if you have a private LiveJournal, as many people (not me, but many people) do? This feature now can prospectively tell HUNDREDS of people at a time that you have one.
Again, I believe that, while the ability to crosspost to Facebook and Twitter is a good idea in general, and I appreciate that the default setting for this feature is the "off" setting, I would prefer that the ability to crosspost any comments made to any screened or locked posts on LiveJournal not be available to anyone.
And, just to be clear as glass, my personal policy on this is... Please, if it is a PUBLIC post, go right ahead and repost your comments to any site you choose, at will! My public posts are just that... PUBLIC POSTS FOR PUBLIC CONSUMPTION. I have no problem with them being spread around for more to see. HOWEVER, while I do often invite comments on my (few) screened or locked posts, I ask that they not be crossposted to any other website without my express permission first. Doing so may be grounds from banning from my blog. Thanks, everyone!