It's a phenomenon which seems to have come from nowhere, but in fact computer-based social networks have been around for decades. In this three-part series the BBC's technology correspondent Rory Cellan Jones traces the hidden story of social networking, from the early days of computing and the 60s counterculture through to the businesses worth billions today. From their roots in utopian experiments in California, online social networks spread around the world. In the past few years companies such as Facebook and Twitter have captivated millions of users. But what will be the next big thing...