Company 1 would be a Social Networking and Hosting PaaS from hardware, networking, bandwidth, databases, serverless workers to services like ads, anti-evil, localization, social graph, development tools, frameworks, runtimes. Company 1, would be THE place to build a social network, if you were an upstart. Company 1 could instantly be a tier 1 competitor to AWS and Azure. Imagine being able to tap into Facebooks Identity services, but running your own Directory Tenant, instead of tied to the FBID. Company 1 would compete with Firebase and restart Pulse, it would jump headfirst against Microsoft and its Visual Studio/GitHub offerings.
Company 2 become clients, consumers, of Company 1 services. They include Messenger, WhatsApp, FB, IG, and Occulus.
The other beauty of a horizontal breakup is that Company 1 lowers the barrier to entry for new upcoming social networks, without being a risk to them being eaten by Company 2. Mark then also can profit off selling his competitors service, similar to AWS profiting off Netflix.