WhatsApp has spawned a new generation of instant messaging superusers, with some teenagers and young adults using their smartphones to despatch up to 100,000 electronic messages a year, according to a study by Deloitte.

The number of instant messages composed in Britain in 2014 is forecast to double this year, reaching 300bn by Christmas, after 160bn were sent last year – with some young smartphone owners making the most of almost free services such as WhatsApp, iMessage and Facebook Messenger to send more than 270 updates on a typical day.

With many messages as short as a single emoticon – smiley faces and pulsating hearts are particular favourites – the number quickly adds up. The rapid to and fro of instant messages often replicates phone conversations or face to face chats, but in written form.

The trend, says Deloitte technology, media and telecoms research head Paul Lee, is being driven by young love. "Teenage romance is appropriating technology for its needs," said Lee. "A constant among humans is courting and they use different tools to do it. It used to be hanging on the phone, now it's instant messaging."

The average person sends seven text messages a day, researchers found, but for instant messaging the average rises to 46 a day – about three per waking hour (assuming a typical person stays awake for 16 hours).

In a mobile report to be published next month, Deloitte reckons a small percentage are superusers, with about half a million young people in Britain sending thousands of messages a month.

The volume of instant messages (IMs) overtook the old fashioned text last year – with 145bn texts sent. This year texting will begin to decline for the first time since its invention in 1992, with Deloitte forecasting a fall to 140bn.

Younger users are gathering around the virtual water coolers offered by WhatsApp, or photo-based services such as Snapchat and Instagram, because at a fraction of the price of texts, they facilitate group conversations, allow the writer to decorate their sentences with brightly coloured stickers and emoticons, and send photos or videos.

The revolution in communications is being driven by only a quarter of phone owners. In a survey conducted in May, Deloitte found 30% of smartphone owners and 25% of all mobile phone users reported using an instant messaging application in the previous week. By contrast, nNearly 90% of smartphone owners had sent a text and nearly 80% had made a voice call. Emails and social networks are more popular among smartphone owners, with nearly half of those questioned having used those services from their mobile.

For the minority of those who use them, instant messages are not only replacing voice calls but competing with most types of remote communication. Messaging applications are now big business, and their arrival is threatening to take traffic from social networks. In February Facebook moved to shore up its hold over mobile interactions by paying £11.4bn for WhatsApp, which is growing at a rate of 1 million registered users a day and charges members $1 a year.

"What they are doing with that acquisition is capturing the flow of conversations," said Lee. "Some of their users have gone from talking on Facebook to talking on WhatsApp. By acquiring WhatsApp they keep those users."