First of all, thanks for sharing the love!
To see where all of your credits are stacking up, head to the Billing tab in Settings. There, you should see a Transaction History section. Each signup will result in a $5 credit being applied to your account.
Note to "Forever Free" subscription customers: If you are on a Forever Free account, you will still see this transaction history, but of course your credits will not be debited from your account until you upgrade to