So many webmasters overlook this critical component of a successful site. However, it is one that is easily corrected. A complete course in copywriting is outside the scope of this article, but to point you in the right direction, I would suggest that you start with "Tested Advertising Methods" by John Caples, followed by "Web Copy That Sells" by Maria Veloso.
people who visited the site but did not opt in > you can use negative audiences for your ads with facebook ads, you just take visitors to the site and substract the people that visited the thank you page. For people who did not purchase, you simply can't with affiliate marketing unless the vendor adds a conversion pixel of yours on their thank you page.
Affiliate marketing is also called "performance marketing", in reference to how sales employees are typically being compensated. Such employees are typically paid a commission for each sale they close, and sometimes are paid performance incentives for exceeding objectives. Affiliates are not employed by the advertiser whose products or services they promote, but the compensation models applied to affiliate marketing are very similar to the ones used for people in the advertisers' internal sales department.