My preferred solution to this would be to prevent usernames with illegal UTF-8 characters, rather than change the URL structure.
I think this example happened because you don't have multibyte string support as part of your PHP installation, so Q2A chopped off the UTF-8 string at 20 bytes, instead of 20 characters. This just so happened to split a UTF-8 double byte character in two. You can check my theory in the 'General' panel of your 'Admin' section.
As for the Facebook issue, you don't have to worry because duplicate usernames are checked for (and automatically changed if necessary).