I think virality is a kind metric for the early-adopter stage of something that's not 'marketed' in a professional sense. I remember a time in around 2004 when everyone around me was saying 'have you heard of this thing called Facebook' - over and over again until I eventually (grudgingly) signed up.

That was Facebook's early adopter (or maybe not so early) stage. Now it's established it's the baseline social media platform, but a way to have measured its early 2000s growth might have been to assess its virality.

One way to do that is to look at the number of what I call social media objects appearing - blogs, videos, media stories - another could be the number of people signing up to twitter feeds etc. SLR is a meme, and it needs an ongoing stream of these kind of things to push the virality.