“The combination of celebrity plus social media is a hugely profitable platform for luxury jewellery and watch brands,” says Shaz Moaven, founder of Moaven PR, which specialises in working with celebrities and luxury brands. “Social media increases the reach and visibility of brands internationally. While it provides news stories for journalists, the popularity of the celebrity means that the story has a long shelf-life.”

When Fabergé recently placed its Treillage collection on Olivia Palermo, Erin O’Connor and Andrea Riseborough, the jewels sold briskly. The Fine Treillage Rose Gold ring sold out over Christmas in stores in Britain and America. The brand has found that celebrity endorsement online has translated effectively internationally and, more importantly, into new markets.

A day before the London premiere of The Hobbit, actor Dominic Monaghan tweeted about the Glam Rock watch he would be wearing the next night. “Dominic’s tweet had an immediate impact on sales even before he put the watch on his wrist,” says Moaven. Similarly, when Moaven dressed Britney Spears in De Grisogono jewellery and a Glam Rock SoBe watch in her new music video “Scream and Shout” with will.i.am, the watch sold out within hours. There is now a two-month waiting list for the watch, and the song has gone on to become number one in more than 20 countries.

However, competition is fierce. More haute couture brands, such as Louis Vuitton, Chanel and Dior, now have their own jewellery divisions and celebrity contracts. Companies such as Leviev and Backes and Strauss, which were previously diamond wholesalers, have entered the world of retail. Meanwhile, jewellery and watch powerhouses such as Chopard and Tag Heuer have sizeable celebrity budgets.

Agencies are usually tight-lipped about celebrity fees but Haig Avakian, whose family business has a boutique at the entrance of the Beverly Wilshire hotel on Rodeo Drive, confirms celebrities with more followers on social media sites are commanding larger fees from luxury brands. “From a celebrity’s perspective, their following on social media sites has now become an extra-added value when negotiating certain placements. It provides brands with extra exposure. Someone like Rihanna has over 27m Twitter followers – that’s a direct form of advertising. This is taken into account when discussing placements,” says Avakian.

“Some celebrities and socialites have enormous followings – Pink, for example, has over 12m followers – so the impact is big,” he adds. “The speed at which we can get our brand name out to a massive audience, based on the celebrity’s following, is unprecedented.” When Paris Hilton tweeted about wearing an Avakian necklace at last year’s Cannes Film Festival, for example, Avakian saw a significant rise in the number of Avakian Facebook fans within minutes and the necklace (worth $500,000) was sold out within weeks.

“Product which is seen to work in the entertainment space can have an immediate and profound impact on online searches, sometimes within minutes of the placement – the speed of the reaction can be astounding,” says Vanessa Emilien of RKi 360, a London-based strategic marketing agency, whose clients include Bond Street jeweller David Morris.

However, some brands are starting to re-evaluate their celebrity endorsement strategies, continues Emilien. “The Office of Fair Trading is now asking celebrities to clearly indicate on the tweet that the message constitutes some form of advertising, which for the brand may be seen as counter-productive,” she says.