I honestly think they're kinda banking on piggybacking off of Facebook account integrity systems to avoid the problems that other LLM providers are facing in trying to prevent mass free trial signups for token relays and so forth.
It's not a good system obviously. Google did this as well for Gemini-CLI, but forced it to be linked to personal Google accounts (which caused a great deal of onboarding friction).
They're spending $100 billion on this stuff and they're worried about free trial signups? Genuinely my opinion couldn't be lower about Meta's AI strategy but even I don't believe they're that stupid. They'd 100x rather have that problem than nobody using it.