It seems that in order to trigger the $5 referral bonus, your referee must earn at least $10 cash back.
http://www.topcashback.com/account/Referrals/terms
Note, for the above bonuses to be applied, your friend must authenticate their account and reach a cashback threshold of $10.00 of payable or claimed transactions.
This will probably be easy for those who can do 2 out of the 3 promos but if you can only do 1 because that's the only hotel brand near you, you won't get the bonus unless the room costs at least $100. I am not sure if tax is included or the cashback is paid on just the room rate or any food you charge to your room as well.