I think a successful social media strategy can be summed up in one word - holistic. There are so many channels and different audiences are going to go to different channels. The first step is to understand your audiences. For instance, a bmw dealership may have a demographic of 50-55 year old males, who make $75,000-$100,000 per year, and live in a specific part of town. You want to reach these people through the means they are going to be most receptive. There is a chance that these people are not really on social media and aren't even online that much. So, an online strategy may not even be most effective for this demographic. This is something you should consider for each of your audiences. Perhaps direct mail or even simply tv advertising would reach them both.
If you think your audience(s) are most receptive through online and social media marketing then you want to start with the kind of content is going to resonate with them the best. For instance, 18-23 year old females, in college, who work a part-time job may be most receptive through a promoted tweet or sponsored facebook post. I would start with just a few different types of content such videos and blogs. Then you can expand once you have the next part down. Once you have chosen the content you are going to produce, then you need to select the right channels to share this content through. You determine which channel is right by answering the question, "Where does my audience hang out?"
I believe this is a start to a successful social media / online strategy.
*Note: I understand the examples I used may just be stereotypes and not factual. Definitely research your audiences on your own to find the truth about them!