I think a forum will be the best promotional event. Because a promotion in the forum will definitely reach the target. The forum should be in accordance with what we promote. So will many people who are interested and also visit the promotional links that we make.
Of course the forum is better than social media, according to some books I read, only 10% of people will read and respond to our post, it is for facebook, for twitter is much lower, only about 4% people will respond to our tweets. Forums are more focused and targeted, so forums are better
But there are strict forums that will not allow you to promote your links. Forum owners and moderators are all aware of what you are doing so that they are not allowing it, specially high ranking and authoritative websites. You will only find few forums that will allow you to put up your links in their signature. So still the best for me is social media sites and because you can target your audience and the potential to attract customers is really high.