I have recruited a few friends, but that gets harder as many are just not interested.

I have the same problem. I hardly have any friends to begin with. I'm not even posting on other social media. I had given up on those even before I found HIVE (STEEM at the time).

It can be tricky to build users as you need that critical mass to have enough interesting content

The best path towards retention would be to focus onboarding entire communities at once. Anyone create their own censorship resistant community on HIVE. This is a great selling point to any niche that is fed up with traditional social media.