Most online community platforms offer the admin a means to list members by their join date, the last login, the number of posts they've made, or even the number of hits on their profiles.
You should use this feature extensively:
You can e-mail the most active community members and gradually try to get them more engaged in running the community and converting them to top members.
You can e-mail the recently joined and welcome them to the community.
You can e-mail the oldest members and ask them for their veteran opinion on issues.
You can e-mail the members that haven't visited in ages and let them know what's going on and ask if there is anything you can do to bring them back.
You can approach members that have never made a post and try to persuade them to make their first steps into the community
…and this is just the beginning.
If you're wondering what a large part of your day to day community development efforts look like, this it it. All these micro-interactions will add up into one thiving online community. The effort you put in today will pay off tomorrow.