The title is very misleading. It isn't 73km per second, it's 74km per second per megaparsec.
Additionally the value, also known as Hubble's Constant, has been highly debated amongst astrophysicists due to the massive uncertainties that come with measuring speed over such vast distances.
Also that's not how fast the Earth is moving, it is how fast galaxies are moving away from Earth (using the formula v = H0 D where v is velocity, H0 is 73.8km-1mpc-1 and D is distance from Earth.
I think I'm ready for my astrophysics test next month.....