Viber , the smartphone app that lets people call and text other users free of charge , has recently surpassed 50 million registered users .

提供免费通话及短信服务的智能手机应用viber，注册用户近期已逾五千万。

That may not be the case in other countries , but there , people are already using messaging apps like whatsapp , viber , and skype .

虽然其他国家的情况可能与美国不同，但目前市场上已经有WhatsApp、Viber和Skyper等短信应用。