If a child is on facebook for 5 years (the max possible without violating age requirements and assuming the age of majority is 18) and if harassment events are independently distributed, the chance of this particular child suffering sexual harassment is (at least) 1 - (1 - 0.00005)^(365 * 5) = ~8.7%.

Moreover, given that the 13-17 (inclusive) age range represents a small portion of FB's total user base, the 0.005% figure is a deep underestimate itself, meaning the 8.7% is as well.

Everyone is free to their opinion, of course, but "only 8.7% of children on FB are sexually harassed" seems a bit cavalier.