As you would do anything else. Setup database relationship or document schema like mongo. Pretty much the basic.

Stream has an API you can use for the scalability side of this and an easy getting started tutorial [1] . We also have lots of SDK's and example code to help you along your way in many major languages and frameworks. [2]

Generally speaking, you'll need to track users, topics, follows, likes/dislikes, etc., and then you'd set up follow relationships on flat feeds at Stream, and use aggregated feeds for roll-ups like "15 users posted a topic in the past hour" and notification feeds for "22 people up-voted your answer to ..."

[1] https://getstream.io/get_started/#intro [2] https://github.com/GetStream/