Quickly find out who among the people you follow are not following you back

twitter_quitter_lister.rb

require 'twitter'

# You'll need to get this info from your Twitter Developer account
Twitter.configure do |config|
  config.consumer_key       = YOUR_CONSUMER_KEY
  config.consumer_secret    = YOUR_CONSUMER_SECRET
  config.oauth_token        = YOUR_OAUTH_TOKEN
  config.oauth_token_secret = YOUR_OAUTH_TOKEN_SECRET
end

# Grab a collection of ids for your friends and your followers
friends   = Twitter.friend_ids.ids
followers = Twitter.follower_ids.ids

# screen_names is a collection of users who you are following but are not following you back
screen_names = Twitter.users(*(friends - followers)).map(&:screen_name)

A couple notes:

• This will only work for users with <= 5,000 friends/followers. If you have more than that, you need to use cursors.
• Line 16 will only fetch up to 100 users. If you want to implement this correctly, you need to batch users into groups of 100.