So with the new game, and the updates to the Rock Band website, I knew that there were going to be changes to our profiles. 
 
Today I went and looked at my profile for the first time in a while (because before RB.com was having issues pulling my gameplay data).  What is this "Signature Performance" thing?  I've checked the leaderboards for the song that it says is my signature performance, and I'm guessing that the site is having issues because it only shows 15 people having gotten on the 360 leaderboards for vox on that song.
 
And I have looked this up on the RB forums, but their search function is crap.  It keeps telling me   The following words are either very common, too long, or too short and were not included in your search: 
"signature performance"

I have no idea what it's supposed to mean. Mine is for "Wanted Dead or Alive" on drums... I don't even remember playing that song in RB3.  O_o

I wish the search function on their forums worked better.

I can't even get my account on Rockband.com to be created... It keeps telling me that they have Rocked out too hard. It has been a month or so of me trying... =(

" I can't even get my account on Rockband.com to be created... It keeps telling me that they have Rocked out too hard. It has been a month or so of me trying... =( "

Well, my Profile tells me mine is for "Sixteen", which is a song I know I'm the highest ranked for. Check your Leaderboard rankings! It may also just be the song you've played the most.

It wasn't leaderbaord rankings for me.  Mine is "Seven Nation Army" but for some reason only 15 people were even showing up on the leaderboards for that song... and I wasn't one of them. 
 
I found an answer though!