What's your hacker-approved stack for implementing social graphs / modeling followers?

Hey hackers,

I'm looking to implement a simple follow feature on my app where a user can follow one or more users and be notified of new content released by those users.

My app's current stack is AWS + Firebase Auth. So far I've seen DBs with native graph-modeling support like AWS Neptune and FaunaDB but I feel like that's cost-prohibitive for hacking a prototype together.

I'm curious to know what the consensus is, if any, on how to go about implementing a simple, cost-effective and responsive (query speed) social graph for most product prototypes that don't yet have a huge budget to pay for a managed solution (or just don't want to).