Many people don't realise how social media sites like Facebook and Twitter make their money. Most of the money they make is from advertising, but they blend it so well into their sites you don't even realise they're there. They customise advertisers ads to suit their site so people feel like they belong on the site. People naturally click on the ads because they're targeted at the user. Facebook does this best because of the amount of information it has on you. It tracks your location, your age, your likes and much more.