eMarketer estimates there will be nearly 21 million Twitter users in the US by the end of this year, and a sizeable minority of those will use the service at least in part to follow brands.

Research on Twitter users from Constant Contact and Chadwick Martin Bailey put the share following brands at 21% of the total. That falls closely in line with April 2011 research from Compete, which found 19% of Twitter users found brands to follow on the service.

In the Constant Contact/CMB study, most brand followers kept up with just a few favored companies on Twitter. Their top reason for becoming a brand follower, cited by 64% who did so, was that they were already a customer of the company—far ahead of the 48% who did so just to get discounts and deals.

While Twitter followers may already be loyal customers, that doesn’t mean following has no effect on them. Overall, 50% said that after following a company’s tweets they were more likely to purchase from the firm, and among men the share was 55%. An even stronger majority said they would be more likely to recommend the brand to others, at least in the case of a few companies they followed.

If brands what them to do so, they will have to give their followers what they want. Aside from promotions and discounts, that means information—61% follow brands so they can be the “first to know” what’s hot—and exclusive content (36%), along with content they can share with others and pass along via retweets (28%).