Google+ was never huge, but some people did (and still do) use it and are upset that their preferred social media platform is being yanked out from under them. What "happened" to Google+ was that Google initially arrogantly assumed they could leverage their other services to get people onto G+ by forced integration that people didn't want, which created a backlash. Furthermore, their initial interaction model guided people toward sharing posts only with their circles, which meant that from the outside most of the interaction that was happening was invisible, which fed into the "ghost town" narrative that the media latched on to. Since they were facing an uphill climb in a negative media landscape they couldn't achieve the kind of growth to become the "Facebook killer" that they hoped it would be.