For some reason the social network plugin was defaulting to showing the first and second name of users and not the username, so I've changed it to show the username now instead. Also, on the edit profile section, the option to change visibility was not possible for the First Name. Now it should be, but you will have to renter the name in your settings. Try entering it and setting some fields to show only for you or friends.

I forgot to mention that the change to show usernames does not retroactively apply to the activity notices on walls because the text in those is generated at the time of the activity. But you can delete individual activity entries and any new activity entries should show the username only.