Not an article, and I have no direct knowledge of this either way, but I would strongly suspect that Instagram migrated off Postgres a while back. Probably to fb-mysql + myrocks, or some other RocksDB based solution.

The compression level is vastly superior to any available Postgres-based solution, and at Instagram’s scale it amounts to extremely compelling hardware cost savings.

Also if they were still primarily on pg, it would be one of the largest pg deployments in existence, and there would be obvious signs of the eng impact of that (conference talks, FOSS contributions, etc).

Bigger-picture: Postgres is an amazing database, and it’s often the right choice, but nothing in tech is always the best choice 100% of the time. There’s always trade-offs somewhere.