If it's valuable, you get an IP lawyer to protect the idea and then help you market it. Generally Facebook does not accept unsolicited submissions.
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Ideas are not protected by intellectual property laws. But if you convert your idea into an invention (which usually requires retention of legal counsel), you can apply for a patent and attempt to license your invention. You need to retain legal counsel for this.
As for Facebook, trust me about this---it is inundated with "ideas" submitted by people who think they will get rich quickly if Facebook accepts it. Often dozens of people submit the same "idea" thinking that they arrived at it independently. If you have a valuable invention, you need to work with intellectual property licensing counsel to develop a strategy for marketing your invention---but don't expect to get a meeting with executives at Facebook.