There are so many factors to consider when measuring revenue generated from social media marketing. Social media is primarily about relationship building and networking, a wholly different function to Search Engine Marketing (SEM). Therefore, looking to value the level of traffic is not always easy - and many have different opinions on this subject.
Comparing "eggs with eggs" is the first place to start
On a simplistic level, if we compare the value of Twitter and Facebook in terms of pageviews and website traffic it's important to take in to consideration the size of following you have at each of the Social Media sites. For example, having pageviews generated from Twitter that are almost three times as great as pageviews from Facebook is all very well but if you find you have 5,000 Facebook fans and a combined 100,000 Twitter followers it wouldn't be surprising to have 3 times the traffic from Twitter, would it?.
So instead let’s look at the individual click-through rates (CTR). Here there is no comparison as usually you'll find there is a much higher return from Facebook. There is about an 80% higher chance someone on Facebook will click on a link than someone on Twitter.
Case Study Examples
When measuring revenue, quality is much more important than quantity and assigning an accurate value amount to this is near impossible. Social Media Today recently published an article and they referenced a company called Chompon who recently did extensive research about this as it relates to their own business.
Here is a key screen shot from their findings:


Online Bookings and Tickets Sales Example
Eventbrite also did their own research on this topic in October of 2010. They found that their revenue (in ticket sales) come mostly from Facebook ($2.52), followed by LinkedIn (90 cents) and lastly from Twitter (43 cents)

Arguably the most fascianting part of the Eventbrite study is the point where people are sharing (or Liking) links and pages. Seeing 60% of Shares and Likes happen post transaction or booking is intriguing but also reasonable as people telling their followers about their purchases and actions is clearly more likely than a passive "this looks good but I ain't buying". To cap it off the 20% more ticket sales per share than a pre-purchase share is definitely worth looking at
So what's the moral of this story?
Start adding your Share and Like buttons to your confirmation pages! And boost your sales by at least 20%