Delete Your Facebook iMore
“Basically, if you took one of the surveys, your data was taken. Worse, the discrepancy between the 270,000 users who participated in the surveys and the 50,000,000 who had their data taken is because Facebook didn’t just allow access to the survey-takers, but to the friends of the survey-takers as well.”
So it doesn’t matter how careful you are with your Facebook account, it also depends on how careful your friends are.