Unfortunately I think part of the lack of growth is that an awful lot of people are, to put it delicately, fucking morons and can't be assed to learn how to use the site. That's not an indictment of anything, just the experience I've had trying to get a few people to join. Only one referral ever went "oh yeah, I recognize that site format, it's just like the message boards I've always used." The others balked and never signed up.
Unfortunately anything that doesn't have a UI similar to Snapchat or Facebook isn't going to be something that people can navigate anymore. Younger people are even less "tech savvy" (or whatever you want to call the ability to navigate a bog standard forum/message board site) than older people at this point.