I was browsing this 9 year old topic for ideas, when I realized that getting new users is not a question for me.
My forum has been quiet for two years. No one posted anything. The negligent admin (= me) didn't even bother to upgrade it. Nevertheless, every now and then someone signed up.
I got an email from someone who was unable to register, which was a wake-up call, because there was a whole bunch of registrations waiting for me to activate. My forum was dead and people still wanted to sign up.
How to get new users is not the hardest question.
Harder than 'the hardest question of all' is this one: how do you encourage new users to start posting? And even harder: how do you encourage them to stay active?
I didn't find the right solution from the Internet.