Establishing a successful community is very hard work spanning a few years – in case it’s not a complimentary (= support) forum to some existing product or service that you sell. In the latter case, you’re going to naturally have active community consisting of your customers.
It took us almost 3 years to make a forum with just 2600 registered users writing ~ 18k posts per month. We had to compete with 3 other forums in a very tiny niche, and we now outperform all of them. Nobody (except our team) believed we would reach this goal 
What we did:
- contests and games for socialising
- markets and maps
- regular participation of our team members in discussions
- a bit of journalism
- a bit of translation from foreign languages
- a bit of politics behind the scenes to talk key people into joining us
- regular improvement to rules and following them
- regular publication of guides and tutorials to help people achieve their goals
- watching for dying topics and reviving them aggressively
- curating content thoughtfully
- stating our mission clearly and giving regular examples of how we accomplish it
If the only value you bring with your forum is the forum itself, you’ll have to do all that work. However, if it’s complimentary to some of your products, it will be much easier.
Also, Don't be fooled by activity metrics – they may vary greatly in different community types