That depends. If you're only participating in a forum to promote yourself, that's spam- not marketing.
But if you're active in a forum and make new clients through networking in the same forum (having proven you know your stuff on a given topic) then "forum marketing" can be very effective. I am new to forums myself, but the first forum I joined actually brought me a significant number of new clients. I don't go around posting weak responses with a link enclosed, but that's not how to truly market any business anyways.