If from worldly view is should be from the number of followers and how learned knowledgeable their followers are same like other organization with we measure how they successful base on these.
For me the success of a spiritual orgaization should come from how benefit people receive from the organization either by their daily practice, spiritual activities ie meditation class, yoga etc and communities activities which lead to the improvement of their life style ie more happy, get along with other, more kind, harmony, and always ready to help other.