There is no reason for the first person to press blue. If no one else has pressed blue yet, there is not yet anyone to save.
The question as written, "Everyone in the world has to take a private vote by pressing a red or blue button. If more than 50% of people press blue, everyone survives. If less than 50% press blue, only the people who press red survive. Which button would you press?"
No where in here does it state there is already someone at stake. This could be a limitation of the character count for the twitter tweet that started it. Nevertheless, it is the first person who presses blue who becomes the person Blue Button Pressers are trying to save. Until someone decides to press Blue for the first time, there is no reason to continue pressing Blue. Everyone could press Red and be fine.