If you are a registered business and you make more than $30K per year, you have to register for a GST account and charge the GST to any Canadian companies that you do business with. Any reputable Canadian company will pay you the GST, but you'll be expected to provide an invoice that shows your GST number, the GST applicable amount and the GST you are charging.
An accountant would be the best to advise you on whether you should register as a business or not, but my opinion is that if you are making more than $30K, you really should be registering as a business. It looks fishy to the government if you don't (unless you aren't declaring anything, which is a separate issue entirely), plus you can start writing-off your expenses.
Like with any other non-internet transaction, the GST is charged to the company to which the service was provided. If the company is a network, they'll worry about whether they need to charge their end client. BTW, delivering leads/clicks/etc. is a service.
Surprisingly enough, the GST department is somewhat aware of the internet business. We had a GST audit a while back and I only had to explain the business model to them once. Considering the overall GST was a very small percentage of the overall sales, they understood the business.
Anyway, as has been said here many times before, your best bet is to talk to an accountant experienced in business accounting.