Mark Zuckerberg Rides NASCAR Shotgun with Dale Earnhardt Jr. ... 'HOLY S**T!!!"

Dale Earnhardt Jr. just made Mark Zuckerberg drop an S-bomb ... taking the Facebook creator on his first ride on a NASCAR track ... and it blew the billionaire's mind.

Zuck hooked up with Dale Jr. at Charlotte Motor Speedway on Facebook Live -- and got his first taste of 160 mph speed!!!

In the end, Zuckerberg called the ride an "amazing experience" ... and even got in the driver's seat for a few laps of his own.