I have no idea what you saw, and I'm not about to take your word for it, because I've seen people convinced that things are 'hacks' just because they don't understand what is going on (e.g., Facebook apps granted permission, weak security settings, etc.).
If this 'Sexy Little Girls' page was previously private, then it could have been made public by a number of means, e.g., a phishing attack on the moderator, password reuse from another site, the attacker joining the group by sharing images, eventually being granted moderator rights as a trusted user, and then making it public.
If you look at undercover websites, things like 'The Silk Road', which was a billion dollar secret drug website, it was very easy to get a trusted position in that website simply by being nice and doing favours for the administrator.
When it comes to a Facebook group of this nature things aren't any different, people can be fooled, this guy: medium.com/p/24eb09e026dd lost his twitter account not because of a specific Twitter vulnerability, but multiple social engineering attacks on other websites.
But that's not the same thing as Twitter, or Facebook, being just instantly hackable.