Hello friends facebook now offer his android user free calling, means you can talk your friends free.So in Facebook Messenger devloper team added a new call feature .
So You can call using facebook social messanger.
Facebook- free phone calling feature will only be used on Android phones . Calls for it to receive and make calls both Android phones is a must have .
How to Call...
Facebook Messenger chat option
select the friend which you want call
Now select free call option .
Facebook Messenger already chatting, sharing photos and voice messages have been facilitated .