"First Post!" the Internet commenter cries! Well, a few years in the past moreso than the present, but the practice persists despite near pointlessness. So I muse:
First post, an intriguing claim; the semantic and syntactic connotations indicate not an active but a reactive communicative burst. Semantically, the author is both claiming and proclaiming status by virtue of his or her immediacy to a content-rich antecedent, such as foundational media or original commentary. Syntactically, the words "First post!" are ordered such that they may be construed as unintentional mockery of the author's semantic intent, using "first" to adjectivally describe a word that implies 'after'.
Eventually a community reaches its tolerance limit and the number of genuine occurrences drops off, with a brief spike in facetious use some time shortly after the stigma of use is widely recognized. Group policing has a more literal effect in "thumbs up/thumbs down" forums like Digg as "First post"ers quickly fall below the default visibility threshold.