"Last week the NSPCC said 52% of online child sex crimes in England and Wales were committed over Facebook-owned apps"
They mean 52% of the known crimes.
They could be technically correct. Most crimes follow a pyramid of sorts with lesser level offenses on the bottom and the impressive stuff at the top (e.g. there are many drug dealers but very few kingpins). They probably have some sort of low level "soliciting a minor" type of crime to criminalize grooming and of course most of that is going to happen over whatever the dominant communication mediums are, Facebook in this case