tedster - 7:53 pm on Feb 13, 2011 (gmt 0)

Here's another strategy that seems to be growing in importance for me: maintaining opt-in email lists.

I've long liked of this kind of regular contact for customer retention, because retention is almost always easier than acquisition of new customers. But you're bound to get some gmail addresses on your list - and Google tracks clicks made from within an email. They are working to measure actual market engagement, and email clicks are one factor that shows it.