According to The Next Web, Voxer is the fourth-most popular free app currently on the App Store, and the most popular free social networking service, ahead of Facebook in second place.
Digital 'smart' journal service Path was the third-most popular social app, while Twitter came in fourth.
San Franciso-based Voxer offers an app that enables users to send instant audio, text and photo messages to their friends, either one-to-one, or in groups.
Just like a walkie-talkie, messages are streamed live as you talk or write, and friends can join live, or listen/read later.
New Voxer users can sync their Facebook accounts to bring up a list of all their friends who are also Voxer users.
The group chat application, which is similar to Whatsapp, is available for iPhone, iPod touch and iPad, along with Android devices.