From what I gather, you have the option to turn on "crosspost my comments to Twitter/Facebook," and they get a link back to the comment. Including to locked posts, including screened comments. I am not sure how this compromises other people's security, other than it gives out a link to someone else's locked post. (Which is annoying, but shrug.) There doesn't seem to be a way to prevent other people from crossposting links to their comments in your journal.

It looks like you decide when you make each comment if you want it crossposted or not; it doesn't seem to happen automatically (but the "crosspost this" box may be checked by default on non-locked posts.)

And Pingback_bot is nifty; if I still posted at LJ I'd be all over it. (Since I don't, I disabled it.) Pingback means:

• If you make a public post, and
• someone else makes a public post linking to yours, and
• you *both* have the bot activated,
• 
  You get a "reply" to your post from pingback_bot that gives you a link to the post that linked yours. (And possibly a fragment of text.)

It's a way of saying "who's linking to me?" In return, when you link to someone else's public post (and they have the bot turned on), they get a comment about your post that links to them.

When it was on before, it made the metafandom swarm post move much faster 'cos people got notified about when & where their posts were mentioned.