Social media is not totally shit, you still can promote on facebook group and on instagram. It's depend on what products that you sell though, if you sell like cosmetics product, you can join a facebook group about cosmetics and you also can endorse somebody on instagram to use your products.
Right. I would say that facebook is the number one social media platform that everyone uses nowadays to promote anything. Millenials are a very good target and they are all active in this sites wherever you go so it's really the best choice even if you are still new in the business industry.
Don't forget twitter though, especially with the new increased character count. Pinterest is also worth using as well.