I find it rather concerning that the user’s email address is used to generate their username.
Just looking down the leader board list it would not be hard to see which of these names looks like an email, and one could simply add a @gmail.com to them all and see which ones send.
Why do this? Ask people for a username when they sign up.
This isn’t some strange thing, everywhere does it.
I personally don’t want my email to be publicly available, and think it’s irresponsible of you to make it so blatantly public, and also up to the user to notice and change it.