Simply Put, Because It Works.
Social networking sites like Facebook, Twitter, LinkedIn (and others) have done an amazing job over the years of collecting information from their users. Information that goes well beyond what people put into their profile settings.
When users on social media like, follow, engage, post, and comment, those activities become part of the vast mounts of information available to marketers using social media advertising.
Advertisers can tap into this information and present information to highly target and highly interested individuals.
Take your advertising and even employee recruiting to the next level using social media advertising. In addition to helping with reach, it can also do wonders for engagement.