I know our honorable founder user17915♦ did a lot to promote the site as it was in its definition and commitment phase - I think some posts on https://www.reddit.com/r/Korean/ helped?
The 'selling point' of this site, as I see it, should be that it's an easily-searchable database of well-written, clear questions about the Korean language, with well-written, clear answers. I think that's still needed on the web; Other sites that have great user-bases don't seem to have the infrastructure that keeps the questions visible and searchable, and encourages people to keep improving the answers.
So though not a direct answer to our question, one thing that we can do is make sure we build on that by
- Asking any interesting questions that occur to us during the course of our Korean learning, even if we already got an answer from our teacher / wife, etc. (Our questions per day doesn't seem to be increasing as much as our visits per day). We are already getting a lot of our traffic from search engines, so our content seems to correlate to things that people are searching for. The more good content we have, the more search hits we'll get!
- Using all our moderation abilities to keep the site tidy - copy-editing, keeping an eye out for duplicate questions, etc.
- And of course, answering where we are able!
In terms of direct promotion, one small thing I think we can do to promote the site in an honest way is to link to good questions here from other sites - e.g. if someone has a good, specific question on Reddit that already has an answer here. Hopefully that won't be seen as a subversion on reddit - that site has many strengths and allows many types of questions that wouldn't really work so well here.
I'll add any more ideas if I have them.
I do think it may be better in the long run for us to grow steadily and organically - so that there's always a good ration of active experienced users to active new users. A sudden influx of new users might be a bit tricky! But we definitely want to get to the point where every Korean learner knows this site and what it's for.