Boon: Mortal Kombat 9 sold closer to 4 million units than 3 - likely outperformed vanilla Street Fighter 4

This figure doesn't factor in subsequent updates, like the disc releases of SSF4 and SSF4 AE v2012. The disc version of Super Street Fighter 4 was reported to have sold 2 million copies world wide.
Regardless, this is an impressive feat for the NetherRealm Studios team, and shows that the Mortal Kombat brand still has a lot of staying power. It will be especially interesting to see if their upcoming title, Injustice: Gods Among Us, can buck the trend of sluggish sales with recent fighters, when it's released in April of this year.
Sources: Ed Boon's Twitter and Capcom's IR page.