So I thought, okay. I have accounts on all of these networks under "Cleolinda Jones," which is not my real name (REALLY?), so I'm a good test case--we can look at what shows up on my Facebook without anyone's alternate username or identity being outed. Except that LJ won't let me connect to Facebook--"Bogus form data." Well, to hell with you too. I disabled AdBlock, NoScript, my firewall--nothing. I hope I can't get in because they're monkeying with the cross-posting code, but I doubt we'd be that lucky. So! I did what I could: I connected to Twitter, set up a dummy entry (filtered to only one other person), and set out to comment on it and see what would show up on Twitter, thereby annoying the piss out of my follower list for a good 30 minutes (start from the bottom):
Subject: Pointless comment spam
Let's see if this shows up.
Subject: So incredibly pointless
Okay, here's a comment that should be longer than the Twitter character limit. I still don't understand why you would want to bore people with this stuff anyway, but there you are. Is it long enough yet? Maybe a little bit longer. There? There.
Last one, I won't bother y'all again, I promise. Let's see what it does when the comment doesn't have a subject title...
And, as everyone reported back (often in great confusion), they were not able to access the entry. Which was good.
So, on the limited medium that is Twitter, 1) a shortened link to your comment is posted, 2) a subject heading only is posted, if you have one, and 3) as much of the comment as will fit is posted if you don't. It won't post the title of the other person's entry, however. But the example in the explanation was "Oh, honey, he's an asshole, please leave him." If any of that is in the comment subject heading or the first few words, and it goes out to your Twitter (accidentally or not), and Honey's boyfriend Asshole is reading your Twitter to keep tabs on you (and thereby her)... that's bad. He doesn't have to be able to get to the entry; in this case, the text of the comment/header itself gives away enough. So yeah, I thought it was a bad idea in the first place, but they're going to have to disable this on all filtered posts, or at least allow you to disable other people cross-posting their comments on your filtered posts.
But here's the thing: I can't get Facebook to connect so I can try to cross-post there (and neither can a couple of other people who just tried it). I don't know if a Facebook cross-post would give away more information, like the title of the entry or the username of the person who posted it, since it's not limited to 140 characters. It might be even more revealing over there. Has anyone tried it?
ETA: Here's what a Facebook cross-post looks like, including your icon and the original poster's LJ URL. Hope your icon is safe for work/family/life...
ETA 2: It's POLL TIME. "Awful idea" is currently leading with 96.1%, FYI.
Also, Facebook just let me in. Here's what it posted of a comment I left on this entry, to confirm:
Note how the second comment did cut off, rather than post in its entirety. However: It's not posting "cleolinda.livejournal.com" up there because that's my (the commenter's) URL; it's posting it because it's the original poster's URL.
There you are.
ETA 3: If you do connect to Facebook, it will show your full name as listed there on your info page.
And now you see why I offered to be the guinea pig.