I got this question in a DM and felt like answering it more broadly. Obviously by founding DEV, I have some ideas on this topic.
It started by focusing hard on "single player mode".
The platform sort of grew from me delivering content and seeing if it would get it shared around social media, etc. This pre-dates even having the dev.to website.
I started the website by interviewing popular developers, like this post:
Here's a trick: People are pretty willing to say yes to being interviewed if you make it easy for them.
Gradually I added more "networky" kind of functionality, but only as it made sense for the number of daily users. There was no sense in having too much functionality early on besides my own blogging, but I just added one thing at a time. This takes a long time even in the best case scenario, so this had to be a solo project until it started looking promising.
I then found @jess and then @peter to co-found the company when it looked like that was actually a pretty good idea, but this was more than two years from its inception. This was also an element of evolution. These two brought in very different, complementary skills into the venture. Whenever we have the opportunity to expand, we look for people who bring in new perspectives and skills.
From there we followed our nose for interesting ways to make the platform more appealing for more people. But it was all with the feeling that it's hard enough to even get your friends and family to actually use things you've built. It's hard to change people's habits, so you have to be truly committed to observing how people react to things, having empathy for different use cases and committing a lot of time to the project.
Anyway, that is what worked for us. I think each situation has different opportunities, but the idea of starting with somewhat "single player mode" and evolving as needed seems like a repeatable pattern in general. DEV is a multi-sided marketplace for ideas which started as a one way flow. Any marketplace could have a similar dynamic.