how can i send challenge to my Facebook friends from my app.?

Hey @Vijay Maybe this Video is what you need.

Complete Explanation

Facebook SDK for unity does not allow the direct communication between users applications. The only thing that resembles what you are looking for and is send from the client is AppRequest (shows notification in Facebook app page) i.e.

  FB.AppRequest(
                 to: null,
                 filters: "",
                 excludeIds: null,
                 message: "run you fools !!!",
                 title: "This is the best game ever!",
                 callback: appRequestCallback
                 );

You can also send it to specific friend. The direct notifications to friends/other users using your app are only working on the web fb builds (and are not even mentioned in reference for Unity (just for Android sdk)) :

Currently, only apps on Facebook.com can use App Notifications. Notifications are only surfaced on the desktop version of Facebook.com.

read more at FB Notification reference and UNITY FB SDK DOC

You maybe could do some trickery mixing Facebook app and some other network communication. For instance Facebook allows you to get friends list, as well as the logged user id, using external communication plugin (cannot think of any by now) you compare the ids, if the remote user id matches any from your friend list you send notification using external communication. Just a theory this way would be to costly, also I am pretty sure you cannot use Facebook data to other things that using FB SDK...