It seems to be a simple enough answer- they are rivals because they consist of the same advertising process. When investigated, however, the two advertising types are very different in their methods to reach consumers and results.
With Facebook, the focus is the amount of traffic. And to be fair, in display advertising, this is a large part as well. But with Facebook, the advertisements are projected based on past searches and information on their users. So, for example, by tracking the pages a user ‘likes’ and follows, Facebook generates ads about those topics. If a consumer likes the pages ‘Tropicana Sun Tan Lotion’ and ‘Beautiful Tan’, then the advertisements that Facebook will show on that client’s page will most likely center around tanning products.
In a sense, Facebook markets by looking at previous trends of a client, and base future marketing on that. This works well, although there is no guarantee or reassurance that the clients looking at those ads are remotely interested in buying the product. Thus, the CPC could be increased while the ROI decreases, or remains constant.
Display advertising, on the other hand, works the other way around. By first doing research on the current consumers of the product and target audiences, ads are created to appeal to clients most likely to buy them. They are then placed on websites that these consumers are high users of, thus generating high traffic and rollover onto the business’s website. What display advertising does that Facebook cannot do is predict future trends. Through the study of the consumers of a specific product, display advertisers are able to predict and help create certain trends, whereas Facebook merely follows the trends after they come out.
In this sense, display advertising would be a better investment for a business to gain a high ROI and low CPM.