Twitter endorsement is a practice by which a Twitter account owner is paid by a company to tweet about a product or brand. The most often Twitter endorsements are made by celebs and somes with several millions of followers can be paid in excess of $10 000 for a tweet.
Twitter endorsement may raise advertising transparency issues when it’s not clear if the celebrity has been paid for. Most national legislations have issued recommendations dealing with Twitter endorsements and other forms of digiatl endorsements (blog posts, Facebook pots, etc.).
For US Market, FTC guidelines recommends to have a clear and conspicuous disclosure in Twitter endorsements by using an #ad hashtag at the beginning of the tweet. It is considered as more clear than the #spon hashtag.