Finally, Most Brands Measuring Social Content Effectiveness: Social shares, “likes” among the the most common measurements | eMarketer
"According to the research, 80% of US client-side marketers measured the effectiveness of their social content, with social media metrics such as ‘likes’ the most common. Usage statistics—daily or monthly active users, for example—fell in the middle of the list. Meanwhile, metrics that could identify business ramifications were not used nearly as much, with financially based measurements such as return on investment and sales landing near the bottom"
While these are simplistic measurements, it’s at least a good start in terms of pervasiveness.