What seperates those newcomers who become active members from those who don't?
Usually, there will be a specific, identifiable, barrier.
For example, newcomers might become regulars if they cross one of the following barriers:
- Interact with 5 people.
- Make 5 friends.
- Participate in 3 discussions.
- Initiate 2 conversations
- Fill out their profiles.
- Are personally contacted by the community manager.
You need to analyze these. Take a sample of your last 50 members. Find the ones which became regulars and narrow down what activities they took when they joined. Once you have identified these, you can focus on bringing people through that barrier.
If you get this right, your conversion rate should increase significantly.