Social networks are fun, and are arguably the killer app of the second decade of the 2000s. Unfortunately, as well as being fun, they are incredibly powerful, and addictive. My seven year old son plays one game, which he loves, but he has to feed his virtual pet every day, or it will become sick and die. On those occasions when he hasn't been able to spend as much time as he would like online, the game tells him "Your pet hates you".
All the social networks try to make it possible for you to create your own networks of friends and contacts, and to make it easy for you convey information to them quickly.
Marketing folks have been quick to recognize the potential for building brand, but unfortunately, the Bad Guys have been equally quick to recognize the potential for building mayhem.