Two weeks ago I joined Twitter, which this month announced its 500 millionth user. To put the figure in context, Tumblr reports 150 million users, Skype 280 million, LinkedIn 200 million and Facebook – wait for it –1.06 billion. If you care to mess around on the Beeb’s rather snazzy calculator, you can find out where you fit on this grand old scale. In a world, as it will tell you, where there are now over 7 billion people 1 in 7 of us has some sort of social media presence. The implications of this are vast, and have been much discussed: we are easier to locate, easier to understand, easier to have instant judgements made upon us, than we have ever been before.