Offline the world is filled with a wide range of people some of them nice, some of them mean, some of them dangerous. The same goes for online places.
There are things people can do in social networks to keep themselves safer. Some of these things are practical (like using privacy settings to make sure they are only sharing information with friends) but also behavioural (for example not sharing personal information with people they don’t know offline).
It’s important for children to recognise that just like offline places, some people online can be dangerous and it’s important for them to tell you or another adult they trust if someone says or does something to make them feel uncomfortable or frightened.