Texting apps are taking away revenue from U.S. carriers, estimated by market research firm Ovum to have reached $23 billion by the end of 2012.

While estimates vary as to the real cost of a single text, experts place it as much as 20 cents to send or receive, but when carriers handle the SMS, it could just be one-hundredth of a cent.

Among the apps that are responsible for this development are:

1. WhatsApp - This Mountain View, California company established in 2009 processes 18 billion messages a day, up from 10 billion in late 2012. The app has more than hundreds of millions of users and in Google's Android phones alone, has been downloaded over 100 million times.

2. Line - An app especially popular in Japan and downloaded more than 110 million times, according to Jeanie Han, chief executive officer of Line USA.

3. Imo - An app that provides basic free texting service on instant messaging networks such as Facebook Chat, Google Talk, Skype and AIM by allowing the user to sync his accounts. The free service includes sending of emoticons, photos, videos and voice messages. The app has group chats, push notifications and voice calling the Android and iOS users can tap without using plan minutes.

4. KaKao Talk - Another free app that lets users call and text anywhere and anytime for free. It features support for 12 different languages, fun alternative communications options such as funny voice filters, animated emoticons and walkie-talkie setting, photo and video sending, group chats and rewards for adding a favourite brand as a friend.

5. WeChat - This app provides free texting and multimedia features such as voice messaging, exchange information of photos and contacts with nearby friends and a look around to inform users if anyone nearby is using the same app.

6. Pinger - Its free texting covers over 100 nations around the world. It has a photo-based interface that allows users to pick a friend's photo to view their texts, other pictures, calls and voicemails so long as they are in the same area.

7. textPlus - The app provides free and unlimited text messaging whatever type of phone is used. It offers group messaging and community texting around the world using a chat-room type venue as well as photo messaging and voice notes.

A venture capitalist observed that one result of the shift toward free text messaging apps is major tech companies such as Facebook, Deutsche Telekom and Samsung are likewise going into the messaging app market.

In 2011, Facebook bought mobile messaging startup Beluga, and it even expressed in interest in purchasing WhatsApp which had been similarly approached by Google.

Deutsche Telekom invested $7.5 million in Pinger while Samsung bought MadSmart which makes the popular messaging app Tic Toc. In October, Yahoo Japan acquired a 50 per cent stake in the Japanese subsidiary of KaKao.

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