Not all websites are created equal. So you'll likely just receive general advice that might work for any website. Here are a few tips to add to the list:
Create articles, tutorials, images, videos to help visitors within your niche. You can promote them within your website using something like CTA Featured Threads. This keeps your most important content more visible.
Join similar (not the same) niches and become a well known authority on your niche.
Create topics that are evergreen tips that people might search for.
Look at what your competition is doing. Not just forums.
Search for other ways to generate traffic besides google. There is more to search than google.
Reach out to other websites in your niche and create partnerships.
Pay for advertising.
Other people will tell you to promote on Social Networks.
read feverbee.com and get his (Richard Millington's) book, it has helped me immensely.
Also, put a link to your forum in your signature for all forums. It's free advertising!!! and if you are on other forums as well, then that's a permanent link back to your forum that will always be there.
That's because the newer generations do not use the internet like the older internet generation does. The newer internet generations sit on their butts on their cell phones using social network apps. They do not venture into the "Real Internet" as we know the web to be. You will not be able to get these people to commit to a forum so easily. But, there are ways you can TRY to prevent losing members to social networks. Examples: Make privacy important, make use of forum features, etc. You can also help spread the word about how these social networks collect data from your browsing history to make a profit. And help minimize the importance that people put on social networks rather than their own websites by not making them important to your websites.
Why not join them? Have you thought about just transitioning to running a facebook group instead? The tools don't matter as much as the community, so if the community has moved, head their way!
As far as social media goes, I disagree with CallieJo, social networks enhance a healthy community, not detract from it. How you use social media is the key, though. I use it to find leaders in my industry and invite them to join and share with my community. Reaching out to the leaders / movers & shakers of the niche subject of your community is some of the best use of your time you can spend on social media. Cultivating and maintaining relationships takes time, effort, passion, tact, and commitment. Social media is not to be spit upon, or rejected, it's a tool to be used, as long as you use it well. Wield it like a scalpel, cutting deep into the heart of your topic, finding the valuable people, and bring them to light. (hint, everyone likes to be recognized for their knowledge and experiences, so use that desire to get quality content for your community, where ever it ends up.