hmm, I think this has more to do with active fans turning into passive fans, we've seen this many times.
As the fresh factor of an new artist goes away, after 2 or 3 years, the noise kind of dies down and only the hard core fans are left, people who are neutral decide whether they buy the albums or not base on how good the songs are. What they talk about on a daily basis, are probably the new public figures that are popping out (Rebecca black etc...). Like you said.
But the popularity is still there, it's just that they're not new anymore, Madonna is not mentioned on a daily basis by people, but she has clearly established fanbase, her tours are doing great. Same can be said with Celine's successful las vegas shows.
I think the same is happening with Lady Gaga, when I was in my first year of University, that was 2009, we have had these frosh groups (orientation week groups), my group was p-p-p poker frosh, just when I was absorbing how popular this woman is, I look at the group standing behind us, it was called "Lady Gaga frosh". so yeah,... Now even Gaga does not have that power anymore, so I'd say the craziness has died down a bit for her, like it is with every other artist.