Hi!

I often wondered the same thing. 

There isn't a big community site/forums like this for Seoul. Actually, people in Seoul use these forums and classifieds also. But, specifically related to Seoul, people read a lot of blogs, and some of them are very popular and get a lot of traffic. It may be worth it to advertise on them, or contact the site owners.

www.seoulistic.com

www.expatevan.com

Try a Google search for "Seoul blog" and many popular results will come up. Good luck!