For you to get something for free, the company giving you the item/service has to get something out of it as altruistic business models don't work, particularly when it comes to Internet services.
It's said that Larry Page and Sergey Brin, the founders of Google, wanted to provide their search engine to the world for free but quickly found out that to get it to scale to cope with demand they had to buy hardware and upgrade internet connections and of course pay for more power and appropriate accommodation for both equipment and staff so they had to come up with a way to generate money so launched an advertising operation.
Facebook's infrastructure is similar in size to Google's and although they design their own hardware and have it manufactured by a low cost contract manufacturer, they still have staffing, office, datacentre, power and connection costs, as the customer won't pay for the service they have to utilise the data they have to target ads.
If you are signing up to an Internet service that isn't asking you for a fee, just remember that, YOU ARE THE PRODUCT.
