Mark Zuckerberg explains that the key difference between Facebook and Alphabet is that it 'can't fail'
"There are different ways to do innovation," he said. "You can plant a lot of seeds, not be committed to any particular one of them, but just see what grows. [T]his really isn't how [Facebook] approached this. We go mission-first, then focus on the pieces we need and go deep on them, and be committed to them."
Facebook currently has a tiny set of products - Facebook, Messenger, WhatsApp, Instagram - while Alphabet has Google (which contains YouTube, Android, Search, etc.) alongside Nest, Google Ventures, Fiber, and Google X, among others.
Zuckerberg does admit that this opens Facebook up to an increased risk. If one of Alphabet's companies folds, the overall focus is still there. If one of Facebook's smaller properties folds, it is a much bigger issue. "These things can't fail," said Zuckerberg. "We need to get them to work in order to achieve the mission."
You can read the whole Fast Company interview here.