Now that we're on the important subject of social media, let's make this a trifecta.
Here's Jason Fall's Social Media Explorer blog post on "What is Engagement and How Do We Measure It?" Jason, a veteran PR practitioner, is a bit disenchanted with the term "engagement," feeling that it simply ask if you want to buy stuff with the prospective customer simply asking what you have.
Getting prospects to respond online takes a different skill set than it does to talk with them directly, so "engagement" is a good term for what needs to be recognized and learned in using social media.
Jason's post comes packed with insights. (We're continually amazed at how much time, insight, and creativity given writers devote to and insert into their blog posts for people they'll never meet or talk with directly. This truly illustrates the Web's nature as a universe of social communities -- communities of messages and online generosity rather than handshakes, actual voices, and tippling.)
Jason Fall addresses, or lets others address, engagement issues like measuring, prompting, targeting, and leaving feedback, plus inks can be found to earlier posts on the subject. Overall, it makes for an invaluable piece.