Sometimes active online communities can have a dry spell of activity. They've been active in the past, but participation has dropped off and it can become a self fulfilling prophecy that nobody participates because participation is low. How can I help boost activity in such a community and get people back involved again?
There are surely many approaches to this problem and no "silver bullet" solution.
The way I would start my approach is by considering the distribution of contributions among community members. Are relatively few members providing the majority of the content? Or, are the contributions of the most active member of the community not that different from the contributions of the least active member?
Many internet communities fall into the former category, with a "long tail" of users who contribute very little or not at all. They may be referred to as "lurkers" when they're consistently present or "guests" when they're more transient. Their importance to a community can be hard to discern without introspection tools; how often are they "converted" to contributing users? Do they click on ads? Do they talk about the site to their friends?
If you have some reason to believe that you have a large passive audience with a low rate of conversion of audience members to active/contributing users (and preferably a tool with which to confirm that belief), that might be a sign that your community has enough visibility and you should focus on encouraging your existing passive audience to contribute.
One way to do this is by adding new features. A new feature could be some form of gamification, like the reputation point system here on Stack Exchange. It could be a usability improvement, like giving people the ability to log in with credentials from a social media site and instantly share content between the community and the social media site.
Another route to take might be to organize an activity or event. For example, a monthly "best photo" contest on a photography discussion forum or an invitation for community interviews, a la reddit's IAMA/AMA traditions. Collaborative story or song writing. Whatever it is, it should ideally support the main goals of the community, and present to the passive audience member some added value in exchange for their participating more actively and contributing content.
On the other hand, if you have a small passive audience with a high rate of conversion, that might suggest you're experiencing the normal ebb and flow of an online community, but with an emphasis on the ebb (possibly temporary). Summertime is notorious for its effect on internet communities with strong student presence, either increasing or decreasing participation depending on the community. If most of your community members are active rather than passive, you may stand to gain more from increased visibility through outreach and/or advertisement than by adding features or activities.
Especially in a smaller community, during the low part of such a cycle, there can be some risk of permanent damage to the userbase when returning users see very little new activity or contributions in their absence. That puts each returning user in the position of first actor, the one who has to "get things started up again" after the lull. This can be emotionally burdensome to some people, and a barrier to re-entry after an extended absence. On the other hand, if the community shrinks, the influence of individual members becomes greater; working directly with a few top contributors to focus on providing that energy and welcoming back returning users is probably more effective in a smaller community than in a larger community.
As an aside, Stack Exchange may be an exception in that last respect; when a few top contributors are providing most of the high-quality questions and answers, it can be intimidating to newer users whose contributions receive fewer views, votes and comments, and have lower net scores.
The best solution I've found is to find something that is Worth Discussing and post a provocative question (or diatribe) about it. Folks respond to activity more readily than they initiate it, and giving them something to respond to can bootstrap activity ... which then encourages them to post their own thoughts and gets the feedback loop running again.
However, sometimes things die out because there's really nothing to be discussed. In which case the answer may be to start a new community.