Close, IIRC we cached the fact you had just done a write, and a subsequent read request that arrived on the replica region was then proxied to the primary region instead of serviced locally.
It hasn’t been this way since around 2013 but again I am fuzzy on how. I think that’s when most such data was switched to TAO, which has local read what you wrote consistency. As long as users landed in the same cluster (and thus TAO cluster) what they wrote was visible to them, even if the DB write hadn’t yet replicated to their region.
FlightTracker postdates my time at FB (ended 2018ish) so I’m not sure how that is used. These systems evolved a lot over time as requirements changed.
I don’t remember anything about writes being batched in memcached and merged in on page load.