@Bright I think it depends on what your forum is about and who your forum is for, who is your ideal member? Who is your target audience? Knowing this will help you greatly because you’ll know how to make advertisements that are specific towards your audience and what platforms your audience will be on or not be on. Knowing your target audience is (and don’t just say anyone who likes… or everyone who likes forums… Make it specific) will help you not only narrow down what platforms your target audience is on but it will save you money because you won’t be wasting your time putting advertisements on platforms that just won’t work.
Also, when it comes to forums one thing to keep in mind is that it does take time, dedication and consistency. People don’t just get 100 members over night or even 100 members in months. One important thing to remember is that it is better to have 5 members who are posting consistently and of good quality than to have 100 members who rarely log on and don’t really add value to your forum.
Good luck with everything =)