SF-LOVERS
The ARPANET mailing list for science fiction fans — the first big list that had nothing to do with work.
It started around 1979 at MIT and quickly became one of the largest lists on the network. It was also a problem: the ARPANET was a defence-funded research network, and discussing Star Trek on it was not what DARPA had in mind. The list was shut down for a while, then allowed back on the argument that it was a useful load test of the network's mail handling.
SF-LOVERS proved early that what people most like to do on a network is talk about things they enjoy. It was moderated for years from Rutgers and mirrored to Usenet as rec.arts.sf-lovers, where the fandom carried on long after the ARPANET itself was gone.