Social networks are increasing in popularity. Market leader Facebook claims to have 700 million users. And that's turning it into an attractive advertising platform for companies.Take BMW. The carmaker has six million fans on Facebook. Travel group TUI is another company trying to harness the reach of social media. The company offers prize draws as ways of attracting new customers, and fostering customer loyalty. Miltiades Arsenopoulos and Marta Grudzinska find out how successful the companies' social media campaigns have been.