I was very active on social media back in 2012 but I don’t remember everyone going wild for the number 998,001. Specifically, the craze came from its reciprocal (ie. 1 divided by 998,001):
1 / 998,001 = 0.001002003004005006007009…997999001…
The pattern shows that the fraction generates every three digit number from 001 to 999. But as the video above demonstrates, this kind of mathematical wizardry can be achieved with other numbers and there some quirks involving omitted digits.