Zachary Seward posts a
new take on user engagement at the Nieman Journalism Lab and suggests a metric that ought to be evaluated is how much content is cut and pasted --- and thus shared.
Other services carry shared content, of course, but new technology is emerging (from such firms as
Tynt) with the Web analytics field to measure which passages in a post are moved to another file.
It suggests there is more to Web success than page views and unique visitors.