Facebook is hoping to rival BlackBerry Messenger (BBM) and Apple's iMessage with its own instant messaging service for smartphones.

The social network has launched the Messenger app for both Apple iPhones and handsets running Google Android that allows Facebook members to send messages to others members or those in their smartphone's address book. Facebook says the messages will be delivered through notifications and texts, "so your friends are more likely to get them right away".

"More and more of us rely on our phones to send and receive messages. But it isn't always easy to know the best way to reach someone on their phone. Should you send an email or text? Which will they check first? Did they even get your last message?," said Facebook engineer Lucy Zhang, in a blog.

Zhang is also the co-founder of Beluga, mobile group-messaging developer that acquired by Facebook in March 2011.

"We think messaging should be easier than that. You should be able to write a message, click 'Send' and know that you will reach the person right away. So, we're introducing Messenger, a new mobile app that simplifies how messaging works, and gives you a faster way to message friends and small groups."

As the app is an extension of Facebook's Messages service, users will be able to access their whole history of conversations whether via email, messages and chats (Facebook's equivalent of instant messages) from their handset as well as when they log on to the social network.

At present, the service is only available in the US and Facebook has yet to reveal when it will hit the UK.

BBM is one of the most popular instant messaging programs for smartphones with 45 million users at the end of July this year. Meanwhile, iMessage will be part of the iOS 5 software that is due to be made available this autumn.