It’s easy to track which visitors actually registered. You can do this with Google Analytics. It’s harder (and much more valuable) to track which visitors became regular members.
You should ask your regular members how they heard about the community. Where did they come from? It might be a year ago, maybe three years ago, but I’m betting this is the sort of traffic you want to encourage in the future.
If they came from a particular magazine, blog or promotional activity target that those magazines, those blogs and repeat those promotional activities. If they came via referrals, encourage referrals.
I suspect you will quickly find a link between how members heard about your community and whether they became regulars.