Why are internet services really free?

[myillusoryentity]

I know about Facebook. The price for that is your privacy. But I've been wondering about other services such as Twitter, Tumblr, Youtube, etc. It seems like nowadays, people almost expect these types of services to be free. The more I wonder why these services are free in the first place, the more I worry about what the 'real' price is to be on these services. Is the internet headed this way? An internet where these "free" services are abundant?

[roderick_young]

Those services are similar in that they build a profile of what you are like, and target advertisements at you. It's like Television showing commercials; the business model can work.

[Jack]

Because they make money from advertising , "however although it was never confirmed but social networks (including Facebook ) are often accused for sharing user’s data including private information and behaviors with a third party ." from UltGate.com