The efficiency of globular cluster formation with respect to galaxy luminosity and mass provides a crucial clue to the formation of globular cluster systems and the assembly of their host galaxies. I will review the importance of GC formation efficiencies (e.g. specific frequencies) in studies of GC systems, and focus on recent results from the ACS Virgo Cluster Survey, a deep, homogeneous HST/ACS survey of 100 early type galaxies in the nearby Virgo Cluster. In particular, I will discuss the new results on the mass- and environmental-dependence of GC formation in these early-type cluster galaxies, and I will show how these observations compare to expectations from simulations.