This is a doozy, and there are an enormous number of opinions for this question. Many think they are the worst thing that ever happened to the world, while others praise their existence.
In the realm of security, they can be the source of extreme data leaks if handled in their default configurations. It is possible to lock down permissions on social networking sites, but in some cases this isn’t enough due to the fact that the backend is not sufficiently secured.
This also doesn’t help if somebody else’s profile that you have on your list gets compromised. Keeping important data away from these kinds of sites is a top priority, and only connecting with those you trust is also extremely helpful.   _{Your Comment}