The answer is simple: Establish quarterly events and hype them up.
Nobody logs in, because nobody wants to play alone; It's a paradox. In the same light, if someone plays every day, they get burned out. The result? The server is empty.
There should be a massive, 24-hour event taking place each quarter; four times per year. The event will be conceived and administered by the old regular folks, and will usually consist of similar events: WW, RW, Racing, Flying, Tanks - you get the idea. Emails will go out. Facebook posts will go out. Prizes will be publicized. Friends of friends will show up. The server will be slammed for 24-hours while everyone plays, laughs and brutally kills each other. And then, silence. For months. People return to their lives. But, if it's known that a similar event will be held each quarter, and the turnout is always incredible, people will start playing outside of events so they can practice for next quarter's event. People will take days off work to show up to the event, if the date is known well in advance. Hell, I'd take a full vacation simply to drive around ten hours per day so I was in-practice for the racing aspect of the event. And if this pattern continues, developers will have more ambition to get "new features" ready for the next swarm of players. The wind is blowing, and we need to bend with it. This is the only thing that'll bring everyone back.
I was reading a thread created a couple days ago, asking us to post our most memorable moments. The answer, for most of us, will be organized events full of our friends!
This is the formula, folks. It just needs to happen.
EDIT: I am more than happy to sit down with management and collaborate on how to implement this.