There are various ways to lie using statistics. Let's assume that Facebook's goal was to reach a total of 1 million teens between 13-18 years old. Now they definitely don't want to come into regulators' crosshairs by only targetting these Teens using Onavo/FB Research app so they opened it to everyone including adults. Now let's say, Facebook got a total of 20 million users for participation through this VPN app. So they are now saying "Ohh, don't worry...only 5% of these users were Teens." which is true but on the other end Facebook did achieve its goal to target 1 million (5% of 20 million) Teens with this app. Not saying that this is what happened here but this may be the way they want to defend their act here PR wise.