Facebook may be too late to the IaaS game which is already a crowded field. I think they could easily do PaaS/SaaS based on their bread and butter domains. Between FB, Insta and WhatsApp they are probably the top mobile developer in the world, handle more user-generated content than anyone and messaging. These are super valuable services. Imagine spinning up a business and subscribing to Facebook's content moderation tools or app deployment pipeline? Those would be extremely valuable and don't have an outright market leader right now.