How do big online retail companies get traffic?

[Jake]

Since these businesses have collected very solid statistics on the value of the average new customer, they can use paid advertising with high confidence that it will pay off in the long run, in some cases that new customer may have to make 2 or 3 purchases before breaking even.

A common tactic, also available to the small operators is Ad Retargeting, in which an anonymous cookie is placed in your browser when you first visit such a site, allowing them to show you ads around the internet beckoning you back, studies have found it takes up to 7 exposures to a brand or product name before some trust it enough to buy.
European regulations and browser design trends may be killing this cookie based marketing method.