I asked this same question at an ATA once. I got an answer back that it is not dead. Basically no major updates can be done to it right now, but minor ones can. So things like new rings and powerups can come out and probably summons too. So think of it the game kind of how it was prior to the major updates like BB, DMS, and Coliseum. For years new areas didn't exist since the release of the game. So honestly there was much difference between updates now and how it was 2 to 3 years ago. The only difference is that I see is devs aren't as active with the players as more. Yeah devs filled players with false promises of new areas during that time, but no concrete areas came out of it until last year. Some games have even come out of maintenance mode, so I personally will never give up hope for zOMG! unless it does get shut down completely. So there are like 3 devs that are helping zOMG! in one way or another currently, just no full time devs right now because it's to expensive to pay that much payroll currently. So I found an interesting article of how maintenance mode is better than shutting down a game completely: http://massively.joystiq.com/2012/04/02/the-daily-grind-is-maintenance-mode-better-than-nothing-at-all/
As others have said, there are a lot of users still playing despite the nature of it's maintenance mode. During peak hours (i.e. American prime time) the BR gate, GB null, PS and Doll Shrine have heaps of users roaming. Oh, and VG people are in many of the farming spots! Don't forget that there are multiple layers on a server so even more people are on.