The point in which someone was once famous or well-known, has ceased being relevant in pop culture - leaving them only to be occasionally referenced to in obscure references or jokes, making them fodder for the "where are they now?" file
This usually follows subsequently after that famous person's final attempt at retaining relevance in the cultural spotlight fails miserably, also known as jumping the shark
. This is especially true for one hit wonder
The quintessential example of this would be Milli Vanilli. Once successful in the early 90's, it was later revealed that the music produced was not their own. After this revelation, the public ceased caring about the group; and they have opted to disappear from the public eye out of emasculated embarassment, thus their fade into obscurity.
Other examples are Spuds Mackenzie
, Fred Durst
, Vanilla Ice