Blog comment is the thing where you have to express your thoughts over the article and content. You dont have to use it for the sake of link building only. You have to add your value on that content so other readers can get some idea from it.
Social Media is the only platform in nowadays, from where you can communicate with your audience and you can reach to potential customers. You can give update and news about your website and products to your customers easily. And it is the safe approach as well.