You are very right that leveraging on social networks is actually the cheapest way of promoting your product and services full stop for the most effective way we do not know if social networks is actually the most effective way of promoting your business but it is definitely the most cheapest way of advertising your business.To me online advertising is the most cheapest and most effective.facebook,Instagram and also whatsapp which is almost free or even free because you only use your whatsapp status.there are others like pay per click offered by Google but I prefer Facebook advert.