If you expect social media marketing to drive sales you might be in for a disappointment. The ability of social media marketing to drive sales has more to do with the product or service you are promoting than it does with the effectiveness of this particular technique.
If you are selling plumbing supplies you need to consider that there are only so many people in need of plumbing supplies at any given moment. Sales may or may not respond accordingly so in this way social media marketing is not as effective as some businesses hope it will be.
What social media is most effective at is brand recognition and what I like to call "mind placement."
By having an active social media presence you position your brand in front of the largest possible audience. Combining just Facebook and Twitter alone gives you a potential audience of ONE BILLION people. This in and of itself may not result in an immediate influx of new sales. It does put your brand front and center and places your particular product or service in the minds of people who may one day need it.
Mind Placement means that when they do need your product/service, or hear from someone who does, the first thing that comes to mind is your brand. That's the true power of social media marketing.
Lots of marketers are trying to push you on the idea that social media marketing can increase conversions and for some businesses this is likely true. But not for all businesses. Also, this is social media's most effective use. Once again, social media marketing is a tool which must be leveraged correctly in order to gain the most use from it. Not only that, but you need to have your expectations in line with what can be delivered (and so should your clients.)
If you promise your clients that social media marketing can increase their sales, you are setting yourself up for failure because you cannot control what people will or will not buy, or when. If this were possible traditional advertising would have figured out how to do it long, long ago. Instead, focus on what social media marketing can do, which is to put their product/service/message in front of more people than any other form of marketing. Once you do that, once you are increasing brand recognition and "mind placement" then you can start to analyze why sales might not be increasing.
Because ultimately, that is what social media marketing is good for: getting the attention of audience. After that, all bets are off.