Here is what I know about "Harlem Shake", based on all of the stations I've listened to that might potentially play that song.
I haven't heard "Harlem Shake" on any FM stations here in Saskatchewan yet. I've heard "Down On Me" and "2 Things" on Yorkton's 94.1 the Fox though. Those songs are real urban/rhythmic leaning songs too, not merely pop songs with a short rap verse or edgy synthesizer effects somewhere. So "Harlem Shake" must be considered too polarizing, or something. You either love it or hate it and radio stations must be afraid of chasing half of the listeners away.
"Harlem Shake" still hasn't been added on some of the more conservative, adult-leaning stations in the Midwest and South regions of the U.S. either.
WNCI, however, has started playing "Harlem Shake". They also play songs by A$ap Rocky, Trinidad James, T.I., Zedd's song "Clarity", and that "Gangnam Style" song. So whatever Hot AC lean WNCI once had must be gone now. It's different from the WNCI I remember in the 1990's, where Will Smith was the only rap music played and the playlist was mostly what might be called Hot AC leaning or Alternative leaning.
WNDV seems to relaxed their "no rap" policy a little, at least on weekends. While listening to WNDV online, I've heard rhythmic-leaning songs like Kanye West's "Gold Digger", Outkast's "The Way You Move", and Justin Timberlake's song "Girlfriend" with Nelly's raps left in. They're still conservative with playing some newer stuff though... no "Harlem Shake" yet.
On AOL's Fresh 40 channel, "Harlem Shake" plummeted from being at number 4 to no longer being in the top 20.
Personally, I wish that radio stations would be a bit quicker to play new songs, novelty songs, foreign language songs, instrumental songs, and anything else that is "different". I'm probably not a typical listener though. I actively seek out new, sometimes non-mainstream music and even listen to Internet radio stations from different countries in search of something different. I also understand that some songs are very polarizing. So a "safe" song from a group like Maroon 5 will always get more airplay on the radio than the YouTube videos that have a dance that goes along with the song.