An international network of BBSes which exchanged mail and public messages over dialup connections. For many years FidoNet was the online community; the InterNet existed, but was expensive and/or difficult to get onto.
FidoNet started to die off once the InterNet got popular. By somewhere around 1996, Windows came with built-in networking and WebBrowser (installed by default) and probably the suckiest terminal program on the planet (not installed by default), which didn't help the situation.
The few BBSes who bothered to keep running dropped off after y2k because most of the software and many of the underlying file formats were not 'y2k ready'. Dates folded back to 1900, entire packets of new messages were tossed (in this context, tossed refers to the action of placing incoming echomail in the right place'') and immediately expired. It just stopped being fun after that.