See in this article from The Guardian (UK) what Facebook says about their use of Machine Learning:
Life inside Facebook: how head of developers organises 500 people
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You said earlier that you’re focussing on machine learning. What’s that for?
Deciding what stories to show you when you log on and look at your wall, for example. There’s only a limited number of stories we can display there, so we need to know based on what you like and what there is what to show you. Or to figure out what ads to show you. And for security, to detect whether somebody is sending out a suspicious number of friend requests, or spamming people, or sending spam links.
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