Mobile-messaging app WhatsApp is halfway to fulfilling Facebook CEO Mark Zuckerberg's prediction of one billion users.
The company, acquired by Facebook in late February for up to $US19 billion ($A20.35 billion) in cash and stock, says it has passed the 500 million user mark.
"In the last few months, we've grown fastest in countries like Brazil, India, Mexico and Russia," says co-founder Jan Koum in a blog post.
"Our users are also sharing more than 700 million photos and 100 million videos every single day."
Koum boasted in February that the service had about 450 million monthly active users and about 320 million active daily users.
About one million new users have been said to join the service each day.
WhatsApp enables users to send messages to and from a range of mobile devices including Apple iPhones and Google Android phones.
It does not sell advertising and has very little revenue, charging users nothing for the first year of use and $US1 ($A1.07) per year thereafter.