Once popular groups that no one cares about anymore
It's no secret that If you're a one hit wonder, a severe and sudden drop-off in the size of your fan base is inevitable. But some bands manage to fade completely into obscurity despite having a significant period of popularity and a fan base that once took them seriously. In some cases, this even happens to bands who had more than one popular album. And furthermore, while a band like The Offspring isn't quite what you'd call the most relevant, they've at least maintained a sizable base of hardcore fans. Yet, this isn't the case for other groups. Here are some that I can think of:
Hoobastank
Velvet Revolver
Bush
I honestly don't know one person that remains a fan of any of these bands despite them having success over an extended period of time.