I do see it as an issue--I've seen several new and even formerly active posters that seem to disappear or slip through the cracks and don't come back, either because they don't see value in active participation on FlyerTalk or because someone said something they perceived as snarky or rude that turned them off.
I think that implementing at least some version of the FlyerTalk Ambassador program would be a great launching pad for any efforts to improve member retention. Having designated people whose duties are (and I'd like to perhaps formalize some of those duties) to ensure FlyerTalk is the warmest, most welcoming environment possible will help to keep the conversation positive, helpful, and community-focused.
Building on that, we'd have to check with the FlyerTalk administration and the Internet Brands IT department, but there may be some technical things we can implement that will allow the forum software to provide notification to the group of FlyerTalk Ambassadors (or subset of Ambassadors representing the subforum in question) of members who have been inactive for more than a certain period of time (the exact criteria will have to be hammered out, of course). These Ambassadors may then be tasked to reach out to members via PM (and even via their registered email address, if need be) to make sure they're doing OK, offer themselves as a first point of contact if they have any issues or questions, offer assistance with helping them to maximize their own miles and points earning (with the goal of getting them hooked on it!), and invite them to stay connected to the community and perhaps even attend a nearby DO.
I've seen it said elsewhere on the boards and even repeated by another TalkBoard candidate in one of the recent questions that "miles and points is why we come, but the community is why we stay." Ensuring the community is friendly and helpful is one way to help that, but another is to make sure people are aware of community activities. Having things like index stickies in CommunityBuzz!, the Calendar, and the Itineraries board are helpful and good tools, but they (especially the latter two) are often underutilized because they are both relatively hidden and not user-friendly.
Again, it's going to need to be done in conjunction with the technical behind-the-scenes folks at FT and IB, but reworking these to make them easier to use and to give people the ability to set proactive alerts (such as internal pop-up notifications, email alerts, or calendars that can be subscribed to in Google Calendars or in Outlook) when there is activity in their own neck of the woods or is pertinent to their own travels.
At the very least, being able to set forum email subscriptions that are filterable by [yes, three-letter] keyword would be nice, both for Mileage Run Deals and CommunityBuzz!--and this is something that could even be automatically implemented at the time of registration based on the person's profile location as an opt-in (or even opt-out) notification (i.e. "check this box if you'd like to be notified of events and/or good airfare deals in your area"). This could have the effect of bringing people into the community fold without them even realizing they're being drawn into it, which will have the side effect of keeping them around and able to contribute, participate, and grow the community.