Barron,
I think the issue is:
Who were your top referrering sites on the 9th of May?
Try doing that in Awstats on May 15th.
There are very few packages that keep the data in a detailed enough format to give you that data.
If the 9th of May had a lot of signups, you could go back and figure out what happened differently that day. AWStats does give you referrer stats, but, you cannot break it down and look at last Monday's data today.