Sponsored Tweets, Ad.ly and Magpie are some popular advertising companies that pay Twitter users for every ad that they tweet. The advertising rates on Twitter are largely dependent on the number of Twitter followers you have so a Twitter user with 1000 followers is likely to earn more dollars per tweet than someone with 200 followers.
Some Hollywood celebrities (like top Twitter user Kim Kardashian) make a few thousand dollars per tweet but if you wanted to know how much money an average Twitter user is generating from sponsored tweets, here’s the answer:
Sponsored Tweets says their average payout is $10 per tweet and a user usually gets just a couple of offers per month. Magpie says their users can earn three-figure amounts per month, most in the $100-$300 range. Twittad says their average payout is $15-25 per week. None of these payout amounts are enough money to quit your day job over, but they can easily add up to tidy second income for their users. link.
Important: If you plan to add advertisements in your Twitter timeline, make sure you disclose that the tweet you are writing is a sponsored one. I think disclosure is now mandatory as per FTC guidelines.