There will be close to 21 million Twitter users in the US by the end of this year, and a sizeable minority of those will use the social networking service at least in part to follow brands, says eMarketer. eMarketer also cites research on Twitter users from Constant Contact and Chadwick Martin Bailey which puts the share following brands at 21% of the total. Most brand followers keep up with just a few favoured companies on the platform. Their top reason for becoming a follower, cited by 64% who do so, is that they are already a customer of the company- far ahead of the 48% who do so just to get discounts and deals.