The problem with social media marketing is that it breaks the traditional marketing models. For example, in traditional marketing you might create an ad that says "here's my product", this is why it's better then the competition, and buy it now. Then you advertise it on TV, in the newspaper, on the Internet, etc.
Even early internet advertising was based on traditional marketing models. For example, you basically created a traditional print type ad and had it posted on a few sites, then generally a small percentage of people viewing the site might click on it, which redirected traffic to your site. Hopefully this ad would help you make a sale, and then you would pay for the click-through.
With the social media marketing paradigm it's a great deal more murky because it can mean several different things to different people, and there's not just one way to do it. For example, for some people it can mean creating a YouTube video, Facebook page or a Twitter feed (or even all the above).For example, I have a friend that has a restaurant. To leverage Facebook and Twitter he needs to get his customers to like his Facebook page or follow his Twitter feed. I suggested that he create business cards that he hands out with the checks, that say if you like the food and service tell your friends, create a review on Yelp, like us on Facebook, or follow their Twitter feed. Then provide them with all the links to the sites. If you really want to be savvy you can create a QR code that points to a special social media marketing page that allows them to find your other marketing pages on the social networks.
What you choose will vary depending on the market you're targeting, and what they're using (e.g. web sites, video, mobile applications, etc.). For example, more adults may use Facebook then young people. So if you're targeting younger people, you might want to find another medium (such as Twitter or Pinterest).
The only way you know what's the best medium is to look what others are doing in your same market. It never hurts to imitate the competition, the trick is to do it better then them. If there is no one in your niche market that is leveraging social media, you need to consider what your customers are using. If you don't know ask them.
In social media its about interacting with the customer which is more personal vs traditional marketing where you tell them what to do (e.g. buy your product). For example, you asked them to like your business or your product on Facebook or follow your feed on Twitter, or watch and subscribe to your videos on YouTube. This also has a side benefit of allowing you to start a dialog with your customers, or just communicate with them about new product, services or promotions that you're offering.
In conclusion, I want to be clear that there is no one effective way to create successful social media marketing strategy. You need to remember this is an art and not a science, you have to find what works for your market. Also don't be afraid to experiment with different technologies, or imitate what your competition is doing. If you don't know what to do, as I said earlier interview you're customers and ask them how they found you, you might find out something interesting.