"First Post" is a strange phenomenon in Internet discussion groups (particularly Slashdot) similar to a marking of territory. Site participants race to be the first to add a comment ("post") to a new article. These comments have no use and usually do not contain any text other than "First post!" or variations thereof. Their purpose is therefore just to annoy site users and entertain the site trolls by allowing them to race for the "honour" of being on the top of the (chronologically-ordered) list of comments. This has unfortunately evolved into an element of the troll subculture (cf. Slashdot trolling phenomena), and thus plagues all popular user-commentary sites today in which only a few people may create new discussions, and thus there are few discussions relative to comments (it does not usually appear on conventional webboards, community journals, etc).
The phenomenon generally only appears on sites which have attained a certain mass popularity. If the site gets to the point where people are genuinely surprised to see a new article/official thread without any associated comments (usually because it was only posted moments before), then some strange urge will well up in certain people to express the fact that they were indeed the 'first' to add a comment.
