As @prank7 mentioned, our tech stack has been discussed here. But I would like to answer the question asked by @deviantony.
Before Hashnode was launched, we got sponsorship from both AWS and Azure. While AWS is the industry standard, Azure is catching up fast. We wanted to do an "Across Region" (haven't done it yet) deployment and Azure had their DCs in both US & India at that time. Also, the Azure folks were very helpful - so we decided to give them a try. The experience has been great so far.
Regarding infrastructure - We have two Node.js instances running under Azure Load Balancer. There is also Nginx which acts as reverse proxy. Further, these Node processes are managed by PM2 and are running in cluster mode. Because of clustering and multiple node processes we are able to achieve zero downtime while deploying new features.
We use MongoDB as our data store which is a 3 member replica set. As of now I have not felt the need for sharding, but we'll get there soon. We are also using Redis for caching and pub-sub mechanism.
For image hosting and CDN, we are using Cloudinary.
This was the high level overview of the infrastructure. Do let me know if you want to know anything else. 😃