> Basically I’m optimizing for viral growth, otherwise the business probably isn’t worth it. I haven’t launched yet but

If you have not launched yet but are optimizing for facebook-scale, that's not the optimal approach.

I can't comment on your database experience since I don't know it, but the vast, vast majority of people underestimate by orders of magnitude what a database can handle.

If you're not a large public company we all know about (and you're not, if you haven't launched yet), you don't need all the horizontal scale you seem to be building.

I remember joining one company (still a startup but a large one about to IPO). My day#1 briefing was about how this one database was in urgent need of replacement with a dozen+ node cassandra cluster because it was about to exceed it's capacity any second now. That was to be my highest priority project.

I took some measurements on usage and capacity and put that project on the backburner. The db was nowhere near capacity on the small machine it was running on. Company grew a lot, did an IPO, grew some more. Years later I left. That db was still handling everything with plenty of headroom left to grow more.