Slack is still one of the best examples which have built a habit-forming product. It’s also a great example to show, building a “digital cocaine” which composed of many different factors. Simply buying a $50 nicely designed WordPress theme which will not solve your business problem.
BTW, actually I had a similar issue as Satya mentioned, it’s very difficult to search or discover a historical conversation and a message, and then turned out to realise that I have to pay to that work.
This is a great article and a great analysis, thanks Satya