Thanks to the invention of online user profiles, big websites knows a lot and I mean a lot about their users, and they are letting you advertise to them! You can specify the age group eg. 24-36, you can specify the gender, the location within minimum 60 KM radius. The online world is a very big place but you can target the smallest groups of people!
On social platform you can target certain interests and on search engines networks you can know and target their past behavior. Online advertising has got an immense amount of metrics and can be super effective and cost effectively for any service or product if done correctly.