Pretty prevalent... Sex addiction statistics from the society for the advancement of sexual health conservatively estimate that 3%-5% of the U.S. Population suffers from sexual compulsion disorders. This is likely an underestimate becos' it is based on those seeking treatment for sex addiction. Many of those afflicted avoid exposure & cannot be easily tracked. Other estimates are as high as 10%, ( i.e.~ 30millions!).
No good statistics. There are no definitive statistics on this as the term is not fully defined, although it is being considered for inclusion in dsm-v (hypersexuality disorder) and a rough conservative estimate is 3% to 5% in the U.S. Here's a flipstream on the topic: http://goo.Gl/sfgcj.