In this post I'm going to speak very generally about virtual worlds as a whole, but it applies just as much to virtual environments of any scale, right down to a build on a rented parcel.
It might seem like a painfully obvious truism, but a surprising number of virtual worlds fail to realize that their very existence hinges on one thing: Community.
It's very easy for a virtual world operator to get caught up in easily digestible numerical statistics, such as the number of new registrations, the percentage of logins, or the financial turnover in a given period. As useful as such statistics can be for getting a quick-and-dirty sense of activity within a virtual world, it does very little to illuminate the actual engine which drives it. The health of a community is very hard, perhaps even impossible to measure in numerical metrics, but its' importance to the survival of a virtual world cannot be understated. No matter how pretty your content may be, if no one sees your virtual world as their social forum of choice, all you have is a very pretty, very empty virtual environment.
Good content, kept in fresh and abundant enough supply can string a virtual world along for a while as a sort of virtual cargo cult of players obsessed with new content keep returning for more. Ultimately however, this puts the strain on the world operator to stay ahead of demand for new things, lest the cargo cult wake up and realize just how lonely your virtual world makes them feel.
Likewise, good functionality (like integrated games) can keep bodies in a virtual world for a while, but unless the functionality is social in nature, forget it. All you've done is taken a solitary experience that could have been performed on their own computer and moved it into a online environment. Unless you have a darned good reason why it belongs there and not as a local application, your players will wise up and eventually wander off.
So with all of this doom-saying said, how can a virtual world operator determine and maintain the health of a community within their world? Not surprisingly, it requires moving from a passive monitoring role into a active role within your community. Engage with players both as yourself, and as an anonymous visitor. Find out what they like, what they don't like, what they spend their time doing, and what they avoid wholesale. Their answers may surprise. The reason for the double role is fairly simple, people act far differently around a perceived authority figure (the VW operator) than they do around someone perceived more or less to be a equal. Unless a player has a real bone to pick with the operator, they probably won't be as forthcoming face to face as they will be with someone else. For bonus points, roleplay as a new player; the health of a community can be exposed very often by how easily they will accept someone new into the fold.
As an extra step, give the community every possible avenue which makes sense to communicate betwixt themselves and to the operator. It often makes sense to create a web-based forum, a blog for official news, and a Twitter/Plurk/Facebook presence as well. Whatever choices are made, its worth noting that communities are pragmatic organisms first, and as long as they aren't interfered with, they will organically grow communication methods of their own. If they find they can socialize better through these outside methods better than through the virtual world, know that it's not unheard of for a community which coalesces in one virtual world to abandon their original world to move to greener pastures.
The bottom line is this: Communities are difficult, fickle things, but ultimately they are the reason for virtual worlds in the first place. A good community can take a mediocre world and transform it into a gem, a bad one can turn a gem into a ghost town. Ignore the health of your community at your own peril.