Typically you'll need some form of monopoly, close monopoly, or collusion between competing entities. For websites this can be the 'network-effect' cost. You use that site because everyone else you know uses that site.

The web itself is an environment ripe for abuse because, again typically, your end user doesn't pay directly for the services (they are the product). This is pretty much why every large social media site is an advertizer. About the only way to make money pushing web pages is by selling your users to advertisers.