Membership tracking, as any web admin can attest to, is always sketchy at best. How does one measure it? I'll not get into that.
What we do have is a good regular base of members who keep the community rolling along and a dedicated staff there to assist new members and get them pointed in the right directions for what they're into.
When I say that we have at least 200 that check in regularly I was basing that off the 220+ accounts that have logged into our site in the past couple months. Some of our members are active on our Ventrilo server or in games, but never log into their accounts or post on our forums.
As comes with the territory in the gaming industry, people come and go from games, some more quickly than others. In one notorious MMO we recently ventured into (that I'll leave unnamed) we picked up nearly 300 new members in one month only to have that game's branch fall to around 25 active the next. Fortunately for MG, many of those members decided to stick around and play other games with us. A lot of our members also come and go as their own personal interest in a game is piqued. Someone that may not have checked in in 6 months or more may show up again as soon as we start into another game that they've been looking at. It's a fluid process that we've learned to work around and is part of why we're successful.
I can say, with some level of confidence, that as far as military gaming communities go (the few that there are) ours is at the top of the list as far as numbers and presence in games, especially so in the PC market, less so in the area of consoles, but we're working to improve that.
We're in the business of doing the dirty work for our members, sort of like a match-making service. We do our best to ensure that our members have served or are serving in the U.S. military and give them the tools to link up and game with other like-minded professionals. This often includes running guilds (or whatever flavor a game chooses to call it) for our members. We refer to these as our branches.
We're always looking to expand to new games, but to ensure that we don't jump into a game that will flop and therefore cause us to scramble to hold onto members we do our research a bit first.