The origin of cross
posting dates back to the early days of
Usenet and other
similar news/bulletin systems and the way that posts
are made on there (which predates
IRC and IM by many years).
On
Usenet, It is possible to post a message to multiple
newsgroups at a single time by listing multiple groups in the '
Newsgroups'
header. Unless the
followups are set to something else,
a reply to that post will appear in all the newsgroups. While this
originally was used mostly for addressing multiple groups with commonly
interesting material (for example, a newsgroup about
abstract games and
computer
AI), it is now the domain of
trolls and
spammers.
Fortunately, on Usenet, a cross posted message appears as hard links in
the Unix file system in multiple directories, all linking to the single
inode with the message itself. The advantage of this is that it takes
no more space (other than the directory entry) on servers to have a
message posted in one newsgroup or 5 newsgroups (or heaven forbid, 100
newsgroups).