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Facebook has a cash cow. It's called News Feed, and for the past five years, it has been the company's core money maker and source of revenue growth.

But there is a problem looming: Facebook has been saying for the past year that it is running out of places to put ads in News Feed. The company has determined that it can't put more ads into users' feeds without harming their experience. The industry term for this is ratio of ads to other content is called "ad load," and Facebook says News Feed's ad load is all maxed out.