For those of us stuck in the Oughties

What exactly counts as "cross-posting" in the context of LJ's latest snafu? Because it can mean more than one thing. Does it mean if I see a fun post by a friend I can't link to it in my own LJ without (potentially) causing mayhem of some sort in FB or Tw? Everyone's all-a twitter about LJ's latest snafu, but I'm still not clear on exactly what it is I'm not supposed to be doing if I want to be a good netizen.