There's nothing traditional about social media, so looking at traditional metrics (or metrics traditionally) will not serve you well. Here are just a few of the value indicators that will help you see the return:
There's no customer like a current customer. We all know that if you pluck two people off the street, the person who's already done business with you is more likely to do more business with you in the future than the person who hasn't. Consider your Facebook fans, your Twitter followers, your LinkedIn contacts, etc., to be existing customers. Why else would they be opting in to a social media relationship with you?
One company's fan-base is another company's database. As noted above, these are customers. Treat them like customers. That means they deserve to be treated differently than the person who's never done business with you before. Don't run the same coupon on Facebook that you run in the newspaper (unless you're trying to insult them). And don't just use social media as a coupon distribution tool. Give them offers and invitations that you can't (or wouldn't want) to give to just anybody. Ask them about their preferences, share news about your company with them in advance, engage with them the way you would if they were standing in front of you at the point of purchase.
Participation is not the only metric of success. Somebody much smarter than I (Forrester Research) discovered that the social media universe is subject to behavioral segmentation just like the consumers of other forms of media. When asked to define the nature of their engagement with the medium, this is how social media users responded:
*Source: Forrester Research NACTAS Survey
The bottom line is that the vast majority of people engaging with social media are not likely to be typing replies to your posts or tweets -- nor are they likely to create a missive in response to your extraordinarily well-thought-out blog. For them, just being there and watching everything that's going on around them is how they engage. That being said, for every comment posted, there are two to three lurkers. So be sure to take every comment seriously. People are watching.
Take a broader view of ROI. Don't limit your definition of ROI to that week's sales. I have a client who sees a lift in sales six weeks after each social media initiative. Why six weeks? Who knows, but it happens like clockwork. In your industry it could be two weeks, four weeks or eight weeks later. The point is, it's not about today.
Use conventional metrics unconventionally. Use conventional brand tracker metrics to validate the value of your social media devotees and include them as an audience segment within that study. Purchase intent, average purchase, purchase frequency, brand perception and brand promoter scores should all be significantly higher against this segment than virtually any other group -- allowing you to feel much better about that social media budget allocation.
"No purchase necessary". You don't have to be a retailer to know that not every customer buys your brand every day. Wouldn't you love it if your best customers stopped by every now and then just to check out what's new, chat with fellow customers and just feel good about being your customer? Letting them come in and wallow in your brand experience helps keep you top of mind, affirms previous purchases and makes them more likely to buy when the time is right. Done right, that's what social media provides.
By using/maintaining dialogue with your very best customers through social media you will keep your brand top of mind. That in turn can only help increase traditional brand health scores on a longer-term basis ... including the ultimate metric ... sales.