Facebook is a great way to advertise. Also there are a lot of facebook groups dedicated for ragnarok server advertisement but the thing is some server owners will spam their way to bury your ads and there are some who badmouths other server to make theirs look better. So this is a little hard.
Rate My Server is, I think, the best way to advertise but you need a lot of reviews to get your server to the first page. This is also hard for giving rewards to players who write a review is against their rules. Forum is also a great way also.