One thing I have consistently said about social media is that it doesn't perform miracles on its own. Almost every successful use of social media includes other forms of marketing too.
One example is the Foodbank RGV. Great social media use; but it includes a website, television interviews, radio interviews, billboards, special events, newspaper stories, and other traditional marketing methods. Social media ties them all together.
If all you can afford is social media, do it. However, start branching out into other forms of marketing as soon as you grow your budget.