At least with Google, SEO seems a lot tougher than in the past. In fact, I've pretty much given up on it, instead of looking for free traffic from social media and via PPC, assuming I can afford it. Anyway, I still think SEO on Google, and of course Bing - is possible and I think one good method is via guest posting. Also, I think the social sharing of certain articles has some powerful effect.
Oh, by the way, we have to look at three more factors: visitor time on the site, whether or not people are specifically looking for your website, and whether or not your forum asks specific non-common questions people want to know. For instance, a general discussion forum with answers about a person's favorite browser isn't going to rank as high as some forum answering where one can find specific work-at-home jobs or solutions to complex chemistry problems.