I have seen this on several of my friends' pages and I totally agree with them.
Please, don't use the Facebook or Twitter crossposting feature with comments on my LJ.
It amounts to an invasion on my privacy, and I really don't want that. If I want to say something on Live Journal, I will say it on Live Journal, if I want to say something on Facebook, I will say it on Facebook. I don't consider the two to be the same thing and I don't want them being used as if the were. And as for Twitter, don't get me started...let's just say that I think it's absurd.
I have friends on one or the other and a few on both and that's how I want it, but I use them in different ways for different things and that's the way it will stay.
Other people have mentioned removing friends who do this or intend doing this. I just ask that you don't do it on any of my entries, or my friends' entries, and certainly don't even think about it without asking permission first. And the answer will be, 'No', by the way.
I don't want this journal to become totally friends only, but it will if someone uses that facility in my journal.
I'm begging the people who run Live Journal to reconsider and remove this stupid, unnecessary facility from LJ. Don't want it, don't need it, won't have it!
Edit: My wonderful friend from the land of the long white cloud, tatjna , has commented with this. I've just tried it and it seems to work to remove those buttons. People are so resourceful when they need to be. Grateful.