WhatsApp reached 500 million monthly active users as the mobile messaging service saw its fastest growth in markets such as Brazil, India, Mexico and Russia.

The company, which is currently in the process of being acquired by Facebook for $19 billion, added in a blog post that its users are sharing more than 700 million photos and 100 million videos per day.

WhatsApp’s status as the world’s most popular mobile messaging app appears to be secure for the time being, despite impressive growth from rival services.

It has added 50 million users since February’s announcement of the Facebook deal, and recently revealed that the service handled a record 64 billion messages in 24 hours. In January, WhatsApp co-founder and CEO Jan Koum said the company was processing 54 billion messages per day.

Japan-based messaging app maker LINE recently said it had 400 million registered users and is aiming to reach 500 million by the end of the year. Akira Morikawa, the LINE CEO, recently said the aim is to reach one billion users during 2015.

However, the fact that LINE reports registered users rather than monthly active users makes comparisons with rival messaging apps difficult.

Tencent said in March that its WeChat/Weixin messaging app has 355 million monthly active users, while Viber’s most recent user figure is 300 million.

The success of these messaging apps is putting pressure on the SMS revenue of operators as consumers increasingly communicate using mobile internet or Wi-Fi connections.

Juniper Research recently forecast that instant messaging apps will account for three-quarters of all mobile messaging traffic in 2018.

The report suggested that the additional functionality provided by IM messaging over SMS — such as the ability to send messages to multiple users, conduct group conversations and send stickers, emoticons and images — will drive traffic volume for messaging apps.

WhatsApp will soon pose another threat to operators after announcing plans to launch a voice service in the second quarter of the year. Mobile operators still generate most of their revenues from providing traditional voice services.

Although WhatsApp is clearly on a roll, it was recently forced to downplay reports alleging that messages sent and received on its service can be accessed by malicious individuals.