Multi platform messenger services running on many types of smartphone

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I observed that one of the common activities for smartphone users is chatting with their friends. Even smartphone like Blackberry has their blackberry messenger or more popular with BBM that allow blackberry users to chat each other. However, this actually also prevent them to chat with their friends who has different platform of smartphone like Iphone or nokia. They must send texting or short messages services which is not free like those BBM. Until What's app launched their messenger application that becomes a bridge for all kind of platform. People just need to download and install this application and register their phone number and directly connect to their friends who also has what's app in their mobile even though they are using different platform as long as they has internet connection. I found this really helpful to chat with my brother who still fanatic with his windows mobile phone. How about you? Have you ever tried this application? Is it working well in your country?