Meta prioritizes availability over strict consistency for most products.
News Feed: Eventual consistency. If a post takes - seconds to appear in all followers' feeds, that's acceptable.
Messaging: Stronger consistency needed. Messages must arrive in order and not be lost. Use write-ahead logs and delivery acknowledgments.
Likes/Comments: Eventual consistency. A like count being off by for a few seconds is fine.
Know which consistency model each product needs and explain why.