For my writers' forum (http://becreativeforums.com), I decided to start promoting it on book forums through signature advertising and advertising topics (where allowed). From that we've gotten 456 members. Now I'm trying to find out how I can get members from other kinds of sites. I've got a Facebook group and I've considered advertising through Facebook. Only thing is, it's expensive.
Any suggestions? +Rep will be given for good answers. FP Cash if it's exceptional 😀
You don't need to advertise through Facebook that way. Just create a fan page and interact with your members and tell them to invite their friends.
Also have you considered going to local book clubs and telling them about it? Offline advertising is always a possibility that people don't really use.
The best I can tell you is to go to other forums the same (or similar) genere to what yours is and put your link in your signature if that's allowed. Otherwise, the only thing I can think of off the top of my head is to request packages, post exchanges, and the works.