Which social networking site did you used or using them to promote your forum? Share your opinions.
I personally use Linkedin and twitter for forum promotion. Although facebook sponsored ads are also good for getting engagement but if you are asking for free methods then its obviously linkedin and twitter.
Twitter works fine for me as I have got about 8k followers. Just the proper timing of the tweets along with appropriate #tags does job fine.
I also believe to some extent that Twitter will work better for forum promotion.
Facebook, Linkedin, Twitter, Google Plus and Pinterest are the best platform for the promotion of social network of Forum promotion.
facebook.com
twitter.com
linkedin.com
pinterrest.com
google plus.com