This question didn't get much attention, even though at that time GLU was 100 days in beta phase. But now, it's 262 days.
Many old users lost their interest and left. If I wasn't highly motivated to keep learning German, I'd leave the site many years ago!
Is there anything we can do about it before it turns to a ghost town?
Maybe we need something like what they're doing on Chinese Language and Usage or what they did on parenting or according to this blog post:
[...] But if you want your site to graduate from beta sooner rather than later, encourage your fellow community members to vote early and often!