If you are starting out don't even think about microservices.

I haven't heard about anybody using Angular for like... 6 years now. Look at React, Vue or Svelte.

You can't get fired for choosing Postgres. Mongo is no damn good.

If I wasn't preoccupied with making social systems intelligent (e.g. use classifiers and clustering to only show you good stuff) I'd be thinking about using websockets to make the site update itself in real time. It would make one heck of a demo (one person posts something, the post appears right away in another browser) but I am not so sure what kind of database you'd use to support that. If you use React or svelte the front-end for that would be easy.