HAVING just received a fancy new watch from TAG Heuer, Cristiano Ronaldo, a footballer, posts a photo of himself, wrist aloft, to his Instagram account. He dutifully thanks them for their “kind gift” and signs off the post with the company’s advertising slogan #dontcrackunderpressure.

This is the latest frontier of a rapidly growing industry. Since January, more than 200,000 posts per month on Instagram, a picture-sharing app owned by Facebook, have been tagged with “#ad”, “#sp” or “#sponsored”, according to Captiv8, an analytics platform that connects brands to social media “influencers”. Hiring such influencers allows companies to reach a vast network of potential customers: Mr Ronaldo has a combined following of 240m people across Facebook, Instagram and Twitter.