The active posting members are a good factor. But, to be honest, they're not something that is necessarily for a forum existence. Why so? Because once a forum gets enough content, like every another website (like a blog or a social media) it's one and the same if there are active posting members or there are just visitors/readers/guests. Because in the world of websites the only thing that matters are the visitors.
In a forum like this one, only 1~2 person/persons are enough to keep it alive. All it needs is to have some new content from time to time and to be spamless. Everything else is okay and we'd like to have it (for example more posting members like Delphine) but it's already a luxury.