Define 'sexually' abused', and even if those percentages were correct over the sample, it's not correct to state them as facts for the entire population.
Sexual Abuse - Occurs when a child is exposed or subject to sexual behaviours or acts which are exploitative and/or inappropriate to his or her age or developmental level.
I've been out of the industry for to long now to quote specific statistics, however 78% does not seem unrealistic.
I really hate it when the community blames the child protection services for letting children down, so to clarrify responsibilities for you:
In order for Child protection workers to intervene, they need a disclosure from the child or sufficient evidence of physical and/or emotional indicators. Consequently it is very difficult to prosecute.
Now there is cross responsibility in child protection - the child protection agency and the Police. Child Protection agencies have a role when the alledged abuser is a parent or family member; has been given loco parentis responsibilities (ie. a babysitter), someone with "care" responsibilities (ie. daycarers); is another child; or the parents are not protective. All other abuse investigations are carried out by the Police. So if the abuser is a teacher, coach, priests, neighbour, etc. it is only the Police who carry out the investigation.
I also strongly agree with Tink, the community is part of the problem. There is no sense of commmunity any more and while everyone thinks it's not their problem and turns the other cheek, it is their inactions that allow the abuse to continue.