This isn't to say that IGNO as a community was dead - IRC was still relatively active, and there was still text messaging and posting in the Facebook group. We still hung out and chatted on Teamspeak (or Mumble - we went back and forth between the two for a while). Even the cbox still saw use.
The main issue was, by 2013, mobile devices had largely supplanted computers as the primary (if not only) means of internet browsing for most of our members. Forum services like Zetaboards felt like they were simply behind the times with lack of any proper mobile website support (a few people made hacky stripped-down skins that looked slightly better on a small screen, but they didn't help much). The lack of push notifications on new posts was also an issue for many in a climate dominated by social media - people were so used to getting automatic updates when there was new content that having to remember to sign on and check the board manually felt archaic in comparison. The time for forums as the dominant form of internet communication had simply passed, and not even IGNO could escape that fact.
I spent a while looking into alternatives, but nothing really clicked - in the end, we were still stuck with Zetaboards. I did what I could to try and make it more mobile-friendly, and even added a notifications plugin (you still had to check the site, but it made finding new posts easier than having to check the dates on the index), but it didn't make much of a difference. IRC got more activity than the forum this year thanks to mobile apps, so I'm hoping I can at least counter-balance this lackluster section with some chatlogs when I have a chance to comb my log folder.