Rejection is hard enough on the telephone or even face-to-face, so why would we want to know who rejected us on Facebook? Well if you're the kind of person that likes to keep score, there are a few apps that will tell you when and who unfriended you on Facebook, or unfollowed you.
For Facebook: Unfriend Finder(http://www.unfriendfinder.com/) and Social Fixer (http://www.socialfixer.com/) will tell you who gave you the axe.
Twitter: Unfollowr (just follow @unfollowr on Twitter, and they'll let you know via DM who quit your 140 words of brilliance), and Quitter (http://useqwitter.com)
Friend or Follow (http://FriendorFollow,com) is interesting. Put in your Twitter name and it will tell you who you're following that isn't following you back, who's following you that you're not following, and who you're following that follows you back.