Word of mouth and referalls. Also one thing I've learnt is not to count advertising as I've spent $100 but got only $50 in sales. Its also about brand recongition (learnt that one the hard way)
Is that $50 recurring every month though?
If so, thats actually a very good amount from only spending $100 on ads.
Brand recognition is the main thing for advertising, it helps spread word of mouth and getting sales direct
from banner clicks is an extra bonus I think.
The word of mouth and people remembering your company name from advertising can be worth a lot more than a few sales from people directly clicking on your ad.