I'm a member of 37 online communities I've joined and no longer participate (that I can recall). That’s a big number.
The majority of these I joined, visited a couple of times, then forgot to keep visiting. These communities had something that interested but didn’t engage me long enough to become a habit.
Joining and forgetting isn't the exception, it's the norm. The majority of people who join communities around the world don't participate after the first few days. They simply forget. That's a remarkable opportunity to boost participate and the number of active members.
When a member joins, you need to keep them visiting, daily, for 3 weeks. If you can keep members visiting your community for 3 weeks you can bet on them to become permanent members.
But you need to plan out a series of activities to take members through this hump. You need a newcomer of the month, personal reminders of upcoming discussions (not mass spam), you need to introduce them to others they might get along with. You need to get them to contribute their opinion on debates. It's really hard work, but the reward is a member for life.
There are plenty more ideas here to keep new members hooked for 3 weeks. It's the biggest opportunity out there. Seize it.