Let’s divide those that come to the community into three categories.
1) Visitors. Those that visit, but are not registered members and don't participate.
2) Lurkers. Those that have registered, visit, but don’t participate.
3) Members. Those that have participated within the last 30 days.
Visitors heard something interesting about your community and decided to check it out. They might visit repeatedly. They’re information-seekers. They quite happily satisfy their information needs.
Lurkers saw something interesting on your platform and decided to register. Yet since registering (or even participating once or twice) they lost motivation to participate. They have reverted back to their information needs. At some stage, the interest that caused them to register was lost.
Members are those whom have made a contribution in the past 30 days. They keep finding something interesting on the platform they wish to participate in. They're trying to satisfy their social needs.
Two thoughts here. First, stop using the number of registered members as your community’s number of members. This is outdated. Use the number of people that participate. By the former definition, most of your members might have completely forgotten about the community yet still be counted as members.
Second, pinpoint the stage lurkers dropped out. Was it after 1 contribution? 2 contributions? 5 contributions? Design interventions at that stage to keep members engaged.
If you know a larger number of members drop out after 3 contributions, or two weeks, you might have an automated message at that stage about something a member can do at this stage. Perhaps you have a ritual after the 5th contributions or after being active members for 3 months. Maybe you give members access to forums for different specialisms or access to the same forums veteran members usually participate in? Maybe after 3 months they can apply to be a moderator in the community?
Or perhaps you simply have a big event every 3 months? There is no shortage of interventions, you just need one that matches the stage members drop out.