func inviteFacebookFriends(){

    let dialog = FBSDKAppInviteDialog()
    let content:FBSDKAppInviteContent = FBSDKAppInviteContent()
    content.appLinkURL = URL(string: "https://example/fbid")
    content.appInvitePreviewImageURL = URL(string: "http://www.queness.com/resources/images/png/apple_ex.png")
    dialog.content = content
    dialog.delegate = self
    do {
        try dialog.validate()
    }
    catch {
        print(error)
    }

    dialog.show()
}