So assuming most are taking a trip somewhere and a trip back, that's about 1.5 billion people involved in this phenomenon, somehow or another- roughly 1 out of every 5 humans on the planet!
This has to somehow include multiple trips per person. Maybe train to airpot + flight + train to destination + the same for return.
EDIT: The official and current UN number is 1.42B. China's last official count was 1.35B in January 2013.
You might also need to travel multiple times to have a traditional meal at the family home, and then go somewhere else for an actual vacation getaway.
Enough edge cases and the numbers could work out. Also the metric is trips, not people; if you live in some provincial village out in the country you may need to take multiple modes of transportation.