Another vote for Koa!

I already had same question. The answer is that social networks by nature are relational this means trying to use a document based DB for a social networks could not be a perfect choice. Of course, you can simulate similar capabilities with Mongo or probably any other NoSQL database but this means a lot more efforts, more hacks and more performance challenges. For example, in Mongo saving comments with post means 2x disk usage (for comments) and one more query for write operations. From other side, MySQL has its own problems but at least it's stable and easy to use, has many available tools and a reasonable performance. So as someone else pointed out here on Hashnode web development is saturated with many tools and technologies do not try to use all of them, do not try to be fancy. Make it work and that's more than enough, at least as long as you didn't create the next Facebook.