Is it efficient to outsource social media marketing ?
I absolutely think it's efficient. To put your social media in the hands of someone who really knows what they're doing will save you a lot of time and, ultimately, enable you to monetize your efforts more quickly.
The *only* circumstance where I would suggest a business may not want to outsource their social media is if they have a very complex product/service in a niche, targeted industry. However great a social media marketing agency may be, they will never know your product as well as you do. For the most part this is ok, as long as they're able to understand the tone and message of your brand. But in some specific cases, this is a challenge that the marketer may not be able to overcome, particularly if you have very high expectations.