It would only apply to top-level posts, and only if the original poster chooses to make the connection. I currently have my journal set to crosspost to InsaneJournal, since a lot of my friends are still over there. This would just give a poster more options of which services they might choose to echo their original posts onto. However, it's still a matter of your post, your rules, and that won't change. You won't even see a tickybox for services that you haven't actively selected to link to; I know there are people who crosspost to JournalFen, for example, but I don't have that tickybox anywhere on my posting form. Facebook will never invade your journal unless you invite it in... and comments aren't part of the crossposting platform DW uses.