I don't use systems that insist on real names. Not because I have anything to hide, but because being listed as "James Jones" makes me impossible to find.
Ideally, it'd be both. First names are nice to have right off the bat--it's a solid/"true" part of a person. But a well-chosen username can reveal their taste, their thought patterns, their interests; it's also a nice convo topic (esp. if usernames aren't displayed by mandate).