It the sites are kept private, as mine is, then there is no problem whatsover with them.
Children should be aware of dangers, as they should in every situation - but there is no reason to stop young adults and children using them.
Not all sex offenders are paedophiles, and there are probably many many paedophiles who have never offended so won't be on any register.
I am more concerned about parents allowing their too young children to dress in a sexually provocative manner, especially in areas such as the seaside which are big magnets for paedophiles.
Why is acceptable to put little girls in push up bras, thongs and tshirts with slogans such as 'so many boys, so little time'.? I know of 9 and 10 year old girls that dress like this.
Teaching children how to be safe on line is a life essential, and Facebook and such like is a safe a place as any.