I don't like the AGPL because it's unclear on this exact sort of thing, but it does seem to me like the obvious reading of "all users interacting with it remotely through a computer network" does not encompass the connection between Instagram end users and their internal Cassandra.

And, in any case, they released sources for the thing they came up with - which is all that the AGPL requires. If they're okay with doing that, they can definitely use the AGPL for production commercial software.