One of our favorite sources of numbers and analysis about social media and word-of-mouth marketing is eMarketer (you see that from how often we cite them as a source on our posts!). The team over there recently published a piece called: "Twitter Users and Bloggers Open to More Than Earned Media"
In the article they cite the fact that may bloggers, social media mavens, and tweeters enjoy the opportunity to promote brands, assuming there is some form of remuneration for the mention:
The authenticity of word-of-mouth has been scaled up by social media and other online tools, but it appears many social content publishers are willing to form relationships with marketers that would move their endorsements from the “earned” to the “paid” column.
In fact, most are already monetizing their recommendations:
As most of you already know, your recommendations of brands, products, and services is a very valuable source of purchasing data to your friends. So keep doing it! Soon this highly effective word-of-mouth marketing will dominate online, thanks to you and all of your efforts!
Also, please bear in mind (as our lessons state!), to be safe, all such paid recommendations/advocations should be preceded with some wording that says that the tweet, post, or comment is in fact an endorsement. This way it is very clear that your recommendation is a paid endorsement that you will be compensated for if your friends buy.