There is a list of them at the bottom of this story.
Facebook's biggest ad clients are one of its most closely guarded secrets. The company makes about $10 billion a year in revenue, almost all of which comes from advertising. But it never talks about who its biggest spenders are.
For the past couple of years, Business Insider has used its sources to come up with what we believe is a list of Facebook's biggest ad clients. Last year, it was probably Samsung.
While Samsung may be Facebook's single biggest marketing client, it's probably not Facebook's biggest media-buying client. That's because Samsung's adspend — like everyone else's — is delivered through a media buyer. Facebook has an ecosystem of media buyers that bundle big corporate ad budgets together. They're called Preferred Marketing Developers.