After we discussed this the other day, I've now changed it so that you can no longer see who a user was invited by. While some people did like it being public, I think the benefits of keeping it public were pretty minor, and there were legitimate concerns about privacy on the opposite side.
So now it's still stored internally and I'll be able to use it to see if someone is repeatedly inviting users that cause problems, but it won't be shown on the site any more. This means that invites are now effectively anonymous—neither the inviter or the invitee will know the other's username if they don't want to reveal it themselves, and other users won't be able to see any relationship between them either.
Hopefully this will help make some people feel less hesitant about inviting others, and as I mentioned earlier in the daily discussion post, you've all been topped back up to 5 invite codes again.