The real power of Facebook is the trove of user data that they obtain through the freemium where the "user is the product". It will be really interesting to see the subscription price and how many users will end up actually paying for the service and to what extent a subscribed user's data collection is stopped is yet to be seen.
In my opinion, serverless is not there yet for large scale latency-sensitive use-cases (where they cannot be hidden by UI tricks). Startup time of lambda runtime (cold container and then the language runtime) is high for web use-cases where the tail latency of multiple seconds cannot be tolerated.
Lambda serverless is really good if you have low RPS and want to pay-for-use due to the low RPS ( prototypes, small production apps, cron-jobs, regular scheduled events, compute intensive - image processing jobs)
One small difference is that the activity on GMail or large email providers is generally two-way and it becomes clear when you understand that clicking "Send" sends your mail away to be stored in GMail forever unlike where photos somebody would be taking with their friends are not aware if it is put in Facebook or similar social places.
I wonder if the equivalent of Github Webhooks is present in Google Cloud Source Repositories, as far as I can see, couldn't find any in the product page.