From http://bit.ly/9YWtAj ...

When it comes to pulling individuals to the website, Twitter behaves similarly to search advertising. Twitter users proactively select the topics, people and companies they wish to follow. When they receive updates that are relevant and interesting to them, and if those updates contain links, those users are likely to interrupt their engagement with Twitter, click on the link and visit the destination website.

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Facebook users, however, exhibit more heterogenic behavior. While some users immediately respond to targeted ads or to inviting links and click their way out of Facebook onto marketers websites, other Facebook users are not in a hurry to interrupt their social activity. In many ways, Facebook behaves as a semi-closed environment, in which individuals engage friends and brands on pages and applications without necessarily extending this engagement outside of Facebook in the short term. Users may eventually arrive at advertisers websites - often via different channels - but not before they have had meaningful interactions with the brands on Facebook.

Marketers, therefore, must view Facebook both as an acquisition channeland track click-through rates to the main websitebut also as a place to nurture and cultivate prospects and customers until they become ready to transact on the main website.