340 thousand people is not that surprising of a following for a band that was notable in the 70's, one that pioneered a popular genre -- it's been nearly 30 years, and you can enjoy a band's music without subscribing to the band's "movement".
I think it's ridiculous to try to make such general and vague pronouncements as "Most Joy Division fans on Facebook must be faking it" with nothing to support or ground your statistic in.