Here is a list of the biggest drops from #1 (by percent) after debuting at #1 on the Hot Digital Songs chart. I'll update as time goes on, but here is a working list of a majority of lead singles thus far.
Taylor Swift - I Knew You Were Trouble. - 62.5% (2012)
1st week: 416,000
2nd week: 156,000
----//----
Lil Wayne/Drake - She Will - 56.1% (2011)
1st week: 255,000
2nd week: 112,000
Britney Spears - Hold It Against Me - 55.5% (2011)
1st week: 411,000
2nd week: 183,000
Lil Wayne/Drake - Right Above It - 55.1% (2010)
1st week: 225,000
2nd week: 101,000
Katy Perry - Part of Me - 54.5% (2012)
1st week: 411,000
2nd week: 187,000
Taylor Swift - We Are Never Ever Getting Back Together - 50.7% (2012)
1st week: 623,000
2nd week: 307,000
----//----
Eminem - Not Afraid - 46.7% (2010)
1st week: 379,000
2nd week: 202,000
Jason Mraz - I Won't Give Up - 44.9% (2012)
1st week: 229,000
2nd week: 127,000
Justin Bieber - Boyfriend - 43.4% (2012)
1st week: 521,000
2nd week: 295,000
Mariah Carey - Touch My Body - 41.3% (2008)
1s week: 286,000
2nd week: 168,000
Kanye West - Heartless - 40.7% (2009)
1st week: 201,000
2nd week: 119,000
Taylor Swift - Mine - 40.7% (2010)
1st week: 297,000
2nd week: 176,000
----//----
Eminem/Dr Dre/50 Cent - Crack A Bottle - 39% (2009)
1st week: 418,000
2nd week: 255,000
Shakira/Wyclef Jean - Hips Don't Lie - 38.2% (2006)
1st week: 267,000
2nd week: 165,000
Jay-Z/Alicia Keys - Empire State of Mind - 36.6% (2009)
1st week: 205,000
2nd week: 130,000
Rihanna - Only Girl (In The World) - 35.3% (2010)
1st week: 249,000
2nd week: 161,000
Rihanna/Drake - What's My Name? - 35.3% (2010)
1st week: 235,000
2nd week: 152,000
Maroon 5/Wiz Khalifa - Payphone - 33.1% (2012)
1st week: 493,000
2nd week: 330,000
T.I./Rihanna - Live Your Life - 32.5% (2007)
1st week: 335,000
2nd week: 226,000
Britney Spears - 3 - 32.5% (2009)
1st week: 255,000
2nd week: 172,000
----//----
Britney Spears - Womanizer - 29.6% (2008)
1st week: 286,000
2nd week: 201,000
Katy Perry - Roar - 29.6% (2013)
1st week: 557,000
2nd week: 392,000
Black Eyed Peas - Boom Boom Pow - 28% (2009)
1st week: 465,000
2nd week: 335,000
Britney Spears - Circus - 27.8%
1st week: 212,000
2nd week: 153,000
Flo Rida/Kesha - Right Round - 27.7% (2009)
1st week: 636,000
2nd week: 460,000
Maroon 5 - Makes Me Wonder - 27.5% (2007)
1st week: 244,000
2nd week: 177,000
Timbaland/Nelly Furtado/Justin Timberlake - Give It To Me - 27% (2007)
1st week: 248,000
2nd week: 181,000
Beyoncé/Shakira - Beautiful Liar - 26.7%
1st week: 150,000
2nd week: 110,000
Rihanna - Take A Bow - 26.6% (2008)
1st week: 267,000
2nd week: 196,000
Taio Cruz/Ludacris - Break Your Heart - 26% (2010)
1st week: 273,000
2nd week: 202,000
Jessica Simpson - These Boots Are Made For Walkin - 25.6% (2005)
1st week: 43,000
2nd week: 32,000
Beyoncé - If I Were A Boy - 23.2% (2008)
1st week: 190,000
2nd week: 146,000
Kelly Clarkson - My Life Would Suck Without You - 22.9% (2009)
1st week: 280,000
2nd week: 216,000
Artists for Haiti - We Are the World 25 - 22.1% (2010)
1st week: 267,000
2nd week: 208,000
Katy Perry/Snoop Dogg - California Gurls - 21.8% (2010)
1st week: 294,000
2nd week: 230,000
Kesha - We R Who We R - 21.4% (2010)
1st week: 280,000
2nd week: 220,000
----//----
Eminem/Rihanna - Love The Way You Lie - 16.3% (2010)
1st week: 338,000
2nd week: 283,000
Rihanna/Jay-Z - Umbrella - 15.5% (2007)
1st week: 277,000
2nd week: 234,000
T.I. - Whatever You Like - 14.1% (2008)
1st week: 205,000
2nd week: 176,000
Justin Timberlake - SexyBack - 13.6% (2006)
1st week: 250,000
2nd week: 216,000
Fall Out Boy - This Ain't A Scene, It's An Arms Race - 13.6% (2007)
1st week: 162,000
2nd week: 140,000
Ne-Yo - So Sick - 12.5% (2006)
1st week: 120,000
2nd week: 105,000
Rihanna - S.O.S. - 12.4% (2006)
1st week: 170,000
2nd week: 149,000
Britney Spears - Gimme More - 11.7% (2007)
1st week: 179,000
2nd week: 158,000
----//----
Sean Kingston - Beautiful Girls - 9.6% (2007)
1st week: 260,000
2nd week: 235,000
Madonna/Justin Timberlake - 4 Minutes - 8.8% (2008)
1st week: 217,000
2nd week: 198,000
Black Eyed Peas - I Gotta Feeling - 6.8% (2009)
1st week: 249,000
2nd week: 232,000
Miley Cyrus - Party In The USA - 6.6% (2009)
1st week: 226,000
2nd week: 211,000
Will Smith - Switch - 5% (2005)
1st week: 40,000
2nd week: 38,000
Kanye West/Jamie Foxx - Gold Digger - 4.9% (2005)
1st week: 81,000
2nd week: 77,000
