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I think there are two distinct areas you need to look at: customer outreach/relationships and brand advertising. The first area, building and maintaining relationships with (mostly) existing clients and customers is where I think social media is most valuable. This can be very difficult to quantify, after all, how do you measure the increase or maintenance of good will through social media contact? Number of likes? I'm not sure how meaningful that really is. The second, which is primarily focused on attracting new business, is much more of a challenge through social media. This is somewhat easier to quantify, through metrics such as Cost Per Click (CPC) and Click Through Rate (CTR).
Honestly, I think the jury is still out on the effectiveness of social media marketing. There is obviously value, but how much remains in question. If you recall, one of the big three auto manufacturers, I think it was GM, pulled back on Facebook advertising over doubts about effectiveness.
Here is a graphic that puts some of the metrics into visual form.
Here's a relatively recent report from Danny Sullivan about that very topic. He writes:
"A new survey shows that when it comes to generating leads and sales, Facebook gets top marks among business-to-consumer marketers while LinkedIn does well for the business-to-business marketer. Twitter comes in as a strong second place winner for B2C leads. The data comes from the 2012 State Of Digital Marketing report from WebMarketing123, a California-based digital marketing agency. ...
[Sixty-seven percent] of B2C marketers who generated leads from social media said some of those came from Facebook, followed by 43% reporting leads from Twitter, then LinkedIn at 21%, followed by Google+ at 15% and Pinterest at 13%."
But, as Sullivan notes, the survey's authors see a disturbing trend for Facebook: The number of B2Cs reporting Facebook as a source of sales fell to 39% from 48% in the previous year. (Twitter was No. 2 in sales at 19%, with LinkedIn third with 9%.)