Thanks I did the correction on my admin panel 
So if I understand well, in fact we are registered as a nickname but we can add our real name. More easy to use the @ I suppose like @omarfilip here. But Facebook can do that with space without problem… 
Btw if I see the “your full name (optional)” on my own discourse’s profile I don’t have this option here (in /preferences/account). Maybe a rank/trusted user matter.
Thanks for your time!