It depends.
Some pieces of your company’s social media can be outsourced (programming, design, etc.). Other pieces should never be outsourced (posting content, administering promotions, etc.). Here’s why…
Social media marketing works best if you are creative (obviously), edgy, and original. For a service provider, it’s incredibly difficult to be genuinely and uniquely creative, edgy, and original for a significant number of clients. Without cheating, they can’t scale—which creates a conflict of interest. This misalignment of priorities (they want scalability, you want a killer campaign manager) yields subpar results.
For this very reason, if you’re an SMB on a limited budget, do not hire a “social media marketing expert” or marketing firm to manage your social media. What do they know about your business? What company-specific news are they going to break quicker than you could? What truly creative campaign ideas will they have (most of these service providers will re-hash the same old strategies over and over again in an effort to scale)?
If you’re a “typical” SMB, don’t be lazy. Learn the ropes. The world is becoming more and more social. Don’t voluntarily pass on such a valuable learning opportunity. Social media is new. The folks that are available for hire are often as clueless as you are. Save yourself some money and invest the time and energy to learn what to do and you’ll be fine.