It is possible to measure the effectiveness of social media by looking at how it helps your customers make decisions. This customer-centric view of social media will give you a better understanding of when and how to use it.
Standard for Success
There is no standard KPI for "success" in social media for the same reason there is no industry standard for what makes a successful website. They are both used to meet multiple goals.
At the end of the day, you want your customers to change their behavior. Whether you're selling cellular services or good ideas, you want them to engage with your company and buy what you're selling. To do that, you have to move them from whatever their opinion is now to one that wants to buy from you. Rather than trying to trick them into it, you have to help them realize how and why doing that is in their best interest. In other words, you have to understand what decisions they need to make and how they make those decisions.
Use your customers' needs and questions as a blueprint for how to interact with them. A customer decision model will help you know the best way to influence your customers' decisions.
Social media won't be effective at addressing every need your customers have. But it might be ideal at addressing some of them.
You can find out where to focus your social media efforts by asking your customers, looking at existing data and by experimenting.
Ask Your Customers
Match your social media usage with your customers' expectations. In surveys and usability tests, ask your customers how they would make some of the decisions in your model. In other words, when they know they have that question, you need to know whether they would turn to social media for an answer. But, just as importantly, gauge how they expect you to use social media. If you use it to send coupon codes, but they expect you to use it to showcase customer stories, you may turn them off.
If your company has been heavily involved with social media and you have a feel for what kinds of decisions your customer need to make, you may already have an intuitive idea of where it makes sense and where it doesn't. Looking purely at engagement metrics (clicked links, shares, retweets, etc.) you can get a feel for what kinds of messages your customers respond to the most. This will give you a high-level indication of which decisions they expect social media to help them with. The obvious weakness of this approach is that it won't tell you anything about an approach you haven't tried yet.
After making educated guesses about what social media should be able to help with, experiment. At a high level, you can test your hypothesis just by looking at engagement metrics. Also pay attention to the feedback you get from customers about new uses of social media. If they mock you for it, you should probably stop.
Social Media KPIs
With this model, you can't just say that social media will result in more sales, unless social media directly influences a decision customers make immediately before buying. But it will indirectly influence sales by helping customers make the next step.
For example, if you are using social media to build credibility and trust in your brand then you would want to see an upward trend in how many followers you have, but you would also want to see that customers are referencing your company as a standard. When you share best practices or new products, those messages should be reshared by followers.
If you are using social media to introduce new products, your primary KPI may be click-throughs. Reshares and new followers are nice, but not a direct indicator that you are succeeding in introducing new products.
The key is to approach social media from the customers' perspective and to understand what direct impact it should have. When we focus on the company goals instead of the customers' needs, we end up with strange KPIs that don't reflect reality or that lead us to do bizarre things with social media. We have to focus on microconversions that benefit the customer instead of insisting on seeing a direct ROI in dollars.