I don't use Facebook, but most site that require usernames are generally the same - you can use anything you want, as long as it's not already taken and it meets any character/length requirements of the site. Some may have restrictions on obscenities or profanities in the username as well.
Sometimes people try alternate phonetic spellings of a name to make it more original to decrease chances of it already being taken - Jawn, Gean, Jahan. Maybe translate it into the equivalent in another language, Ivan, Johan, Juan, etc.
There isn't any style standards for usernames, if that's what you're asking. As long is it meets requirements and isn't taken, go for it.