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For decades, the phrase “Eternal September” has been used to describe the tipping point for a message board or online community—the inclusion, or invasion, of new users who dramatically change the existing subculture. (The term originated in September, 1993, after America Online made Usenet, a decades-old message-board system, accessible to many of its members, who were new to the Internet.)"
Actually, it predates that; referring to when a new crop of undergraduates would arrive at schools on the Internet.