Twitter vs Facebook : Where to advertise ?

A recent study by Nielsen in U.K. found that twitter users violated the Pareto 80/20 principle, which states that 20% of the users account for 80% of the activity. Infact most social networking sites violate the 80/20 rule. Some of them are far above its expectations and some far below.

For Twitter, 7% of the users account for 79% of its activity. For Facebook, 52% of its users account for 98% of its activity. That probably explains the returns on these two social networking sites. A Twitter follow on an average generates $2 and a tweet generates $5 whereas a Facebook like is worth $8 and a Facebook share is worth $14. This dichotomy is perhaps the result of the nature of these two sites.The difference can be seen due different levels of control exercised, degree of self presentation and the type of disclosure on both of them. Although twitter's overlapping nature of service makes it neither a complete social networking site nor a blog. Its ideal nature for celebrities (shown in high adoption and follower levels) and people of importance results in the 79/7 phenomena (Nielsen U.K. study). Facebook is more of a common man driven service as opposed to the celeb driven nature of twitter.
This means that for PR campaigns, twitter might be more suited for organizations as compared to Facebook. On the other hand for a customer engagement and community development campaign, Facebook might be more useful for Digital Marketers.