The thing is, adults continually view video games as toys, when they are now considered to be a valid form of media in their own right with an adult audience- links here give the average age of gamers as 28 and 29 respectively.
http://www.theregister.co.uk/2005/08/15/video_games_and_aggression
http://www.out-law.com/page-5810
Games have carried age classification for 15 years, films for far longer than that. Unfortunately what happens is that shops (correctly) refuse to sell 18-rated games to children, who then nag their parents into picking it up anyway because it's only a game.
The government can't simply stand in for everyone's parents with a constant barrage of legislation in place of common sense.
As for social networking sites, this is all stuff that children have access to at home, or at least around their friend's houses or at school. Navigating difficult situations is part of growing up, and as adults surely we'd be better off letting young people decide what they think is acceptable online behaviour? It's still a relatively new phenomenon, and will build up it's own etiquette style over time in the same way that youtube frantically pulls offensive material, yet resists having to approve stuff as against the creative spirit of t'interweb. (possibly also to give them a leg to stand on in court as well, but hey ho!)