The new service, initially to be rolled out only in the US, will use user’s browser cookies and scrambled email addresses to ‘personalise’ advertising and ‘promoted tweets’ that appear on the site. This means ads will be partially based on your online activities outside Twitter itself to make Twitter's own ads "better".

Explaining the new service, Twitter write in a blog post “let’s say a local florist wants to advertise a Valentine’s Day special on Twitter. They’d prefer to show their ad to flower enthusiasts who frequent their website or subscribe to their newsletter… We can then match that information to accounts in order to show them a Promoted Tweet with the Valentine’s Day deal.”

Given increased concerns over online privacy in light of the PRISM revelations, many are likely to be reluctant to allow Twitter access more information. However, unlike Facebook’s similar service, Twitter users can easily ‘opt out’ of tracking ads by disabling the feature through their account settings page.

Alternatively, users can simply enable the ‘do not track’ feature on their browser (only partially supported by Chrome) that constantly opts out when sites look to use behavioural advertising.