Facebook Messenger launched Tuesday in the U.S. only in an attempt to take on BlackBerry's popular BlackBerry Messenger (BBM).

The move comes as the maker of BlackBerry Messenger, Canada-based Research in Motion (NASDAQ: RIMM) faces controversy over the use of BBM as a tool of choice by London looters and rioters. Research in Motion has responded, however, by agreeing to work with police on investigations to try and pin down the locations of BBM users in the riots.

BBM has more than 45 million users around the world, but while Facebook Messenger is new, Facebook is the world's largest social network, with more than 750 million users worldwide. As smartphone sales have soared globally in recent years, messengers services have grown popular in accord.

Companies including Facebook, RIM, and Google are competing to capture that growing market, and Facebook's effort launched initially in the U.S. is the company's first effort in the space.

The free Facebook Messenger app is available on both Android and iOS devices. Facebook has not said when the app might be available outside the U.S.

On Wednesday, however, just one day after the launch of Facebook Messenger, the app was already the number one most popular in the Apple App Store among all free apps.

Customers had posted 929 ratings already, delivering a 3.5 star rating out of a possible 5.

"Facebook Messenger is a faster way to message," the company's app description says. "Reach friends right on their phones, get and send messages first, message everyone at once and more."

Some users like the product, while others were not so sure.

"I don't even see how this is different than using the actual Facebook app or go online on the phone and message the person!!" wrote one user.

But others liked the product.

"Does exactly what it says it does, wow I forgot what it feels like to get push notifications that work INSTANTLY!" wrote another.

Facebook engineer Lucy Zhang wrote in a blog post explaining that the company's new Messenger app is designed to make the process easier for users.

"More and more of us rely on our phones to send and receive messages," she wrote. "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?

"We think messaging should be easier than that," Zhang wrote.

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