@Walmart, @Cocacola, @Hp, @Generalelectric and @Mcdonaldscorp are the top tweeters between 7.1. – 13.1.2013.
Statistics are based on company tweets (excluding retweets) as seen on companies’ Twitter home page. Accounts are assumed to be official corporate Twitter accounts (other Twitter accounts are excluded, such as division or country accounts). Accounts used in the statistics are listed in this Twitter list
See also this blog post: Top Tweeters Statistics.