Swyft Media of the United States and Asian messaging apps Line and WeChat share this growing niche market with Feeligo.

The latest advertising trend is mainly logo-free, giving emoji users on both ends of the chat more freedom.

"Users are saturated with publicity," Dolle said, adding that many people use free software that blocks pop-up advertising to keep their screens uncluttered by ad banners and videos.

Emoji is "less aggressive, it's not intrusive. We give them playful tools that they can choose whether or not to use. This contributes to making people love the brand and share it in their conversations."

France's state-owned postal service La Poste has also launched its own stickers in the shape of personalised stamps to celebrate special days in users' lives, such as the birth of a child or a wedding.

The campaign has proved a major success, with the digital stamps seen by users more than nine million times, said Rassem Belhouadjeb, a member of La Poste's digital marketing team.

"We exceeded our goal eight times over," he said.

But emoji campaigns may do little to keep brands' foes at bay, as a prank by a graffiti artist in the British city of Bristol showed this summer.

The artist had some fun with one of McDonald's billboards, embellishing it with a vomiting emoji.