Checkout SatNOGS, an "Open Source Global Network of Satellite Ground Stations". These are individuals dedicated enough to set up ground stations to listen to random satellites, many of which are CubeSats. I don't know that they would refer to themselves as a "fan club" but I think it meets your criteria of folks tracking satellites that are not their own. Some people even make fancy mission control like displays for their home or office.
One CubeSat I worked on, MYSat-1 from Khalifa University in the UAE, has been tracked by SatNOGS since its deployment from Cygnus1. Our first contact with the satellite was via a SatNOGS ground station. The satellite has gone quiet twice for months at a time, and surprisingly started transmitting again both times. We only knew of this because SatNOGS stations were autonomously tracking it. The university that worked on it did not have the resources to attempt to conduct full communication passes with a satellite that was deemed dead.
Then there is also the CubeSat Developers Workshop organized by CalPoly. Attendees include CubeSat builders, builder hopefuls, and just overall interested people. Since it is a workshop, it may not quite qualify as a "fan club" but its close to what you are asking.
1MYSat-1 was launched on board of the Cygnus CRS-10 cargo mission and was deployed from the spacecrafts external deployer after departing from the ISS.