According to Official Guinness Records,
The most mentions of a brand name on Twitter in 24 hours is 3,710,044, achieved by Ezaki Glico Co., Ltd. (Japan) and their biscuit stick Pocky on 11 November 2013.
The word "Pocky", which is a name of a biscuit stick product of Ezaki Glico Co., Ltd., was mentioned in Japanese on Twitter as part of their Pocky day celebrations, that take place annually on 11 November.
For a complete list of 2013 records, please visit 2013 Guinness Records in Japan.