Board Thread:Love Live! School Idol Festival Lounge/@comment-26574261-20150705024958/@comment-24187738-20170529025118

So, assuming your game wasn't backed up using the Apple game center or Google play store...

If you don't know your user ID, and you don't have a way of contacting anyone who added you as a friend, there's no way of retrieving it. Profile names are pointless unless someone just happens to come across your profile and given the number of players, good luck there.