tedster - 10:25 pm on Dec 5, 2010 (gmt 0) [edited by: tedster at 10:30 pm (utc) on Dec 5, 2010]
do you mean who clicks on your links from the SERP's or who clicks on your backlinks?
I think it could be both. Google and others are building rather large social graphs. They know who is an authority, who is an influencer. The idea is this: if one of these "important" people - identified by for instance, their Twitter account or their Google toolbar - visits a site, then that could be weighted as an extra signal.
I'm not saying this is true, mind you, but it is worth thinking about. I'm pretty sure that if an "important" Twitter account retweets your link, that means something. Social signals like this are under intense study. Heck, I'm even studying them ;)
[edited by: tedster at 10:30 pm (utc) on Dec 5, 2010]