Word of mouth can be an awesome way to grow a forum, but it works best when it’s paired with other marketing strategies. Word of mouth works because people trust recommendations when it's coming from someone whom they also trust. It also helps build a strong community because when people feel connected they naturally invite others. It also works because it attracts the right crowd and will bring in like-minded people. However, there is a catch to only advertising through word of mouth. For example, it can take forever to grow if you're only relying on word of mouth. Also, if the people you are recruiting to your forum aren't really big into forums or posting your forum growth can stall. Some of my favourite ways to get more people talking would be to make it easy to share, give members a reason to want to invite others, maybe create referral perks or exclusive content to those who refer your forum/website. Team up with influencers in your niche so you can attract the right people and help spread the word. The last way is of course using your SEO to your advantage. Make your posts/threads searchable so people can find your community when they're looking for answers.
At the end of the day, word of mouth is great, but you've got to give people a reason to talk as well as make it easy for new members to find you. I don't think word of mouth alone will work to create an active forum, however it is a very good strategy to use to get some people to join.