Facebook is about to max out on the number of ads it can show users inside its flagship product, which means it will need to find other ways to grow the company's ad business moving forward. Simply increasing the number of ads it shows people will not be an option.
Facebook's user base is already growing. It added 60 million new users last quarter alone. Its flagship product is not the company's only revenue driver. Yes, it's the key revenue driver, but Facebook has plenty of other products where it currently sells ads (Instagram) and will most likely sell ads in the future (Oculus, Messenger, WhatsApp). So it owns a lot more real estate with ad-delivery potential.
Facebook also sells ads inside other people's apps through its ad network, Audience Network and that seems to be working. The company announced back in Q4 that this network had a $1 billion run rate, meaning if you took the network's current performance and kept it consistent for an entire year, it would be a $1 billion business.