Question: I have a strange problem. My ios game integrated with Facebook. I invited to my game a friend, who added to testers in Facebook app, but my friend did not receive invitation. But when he logged in Facebook on his mobile phone, he received invitation immediately. My friend opened browser and tried to find invitation in Notifications but there is no any one.   

Can somebody explain me why it happened?

Answer: 
You have to go to developer.facebook account of yours and make some changes.
Steps:-
1.Select your app from apps.
2.Go to setting option
3.Press Add Platform and add App On Facebook

When Same problem i have occur its solve by this
If your game only supports mobile then your friends will not get notification on their browsers