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What is your reason for advertising? Do you advertise to provide information about products, or drive traffic to your site? Do you advertise to create a brand image so that your product is instantly recognizable?
All of those reasons are right. it depends on advertiser's purpose.
If you are a well established company such as Coca-Cola, then your goal with advertising is to remind customers that they need your product. Also, the social media, even Facebook plays a part in this as you are trying to engage customers beyond that moment when they are using your product.
In our marketing subject in college, advertising is defined as promoting the product or service to the public. Advertising can be personal or commercial depending on the purpose. That means when you post something in social media, you are advertising something whether it is your website or anything because social media is public. My take on advertising is promotion. Anything you promote, you advertise.