How do they make millions giving away services?

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Everyone is familiar with the internet giant Yahoo and host of free services that they provide to the public. Such as email, photo saving and sharing, file sharong, and of course instant messaging and chatroom services. Most of you are also aware that social networking sites (also free) are an explosive niche in which people can market just about anything to their "friends" and contacts on that network. We also know that companies like Yahoo!, MySpace, Facebook, and Twitter are raking in money in the millions and billions of dollars each year. Have you ever stopped to wonder how they manage to get all that money???? I know the answer, I just am curious as to the number of people who know it too.