From looking at the rankings there's clearly some very active people and the userbase is probably not reflected by the activity in the community section. Would it be possible to try to implement website features which would make it more motivating to engage other learners? Make the profiles a bit richer with something like writing a little about yourself and maybe why your're learning Japanese, displaying some stats on how much you've practiced in the last week/month (yes I know about recent activity) and maybe display votes recieved in this section. Possibly adding a somewhat similar feature to let people thank each other in chat or even rewarding them (fake internet points are fine) for communicating in Japanese there?
I'm not saying turn nihongomaster from a learning website to a social platform, but when wanting to engage with other learners to keep motivation up it's somewhat depressing that sorting by activity the 2nd largest group is also on 2nd place in recent activity with a week since last post. Not to mention the chat where the 4 last users wrote something 16 hours ago, 4 days ago, 7 days ago and 8 days ago.