Inre: Facebook/Twitter:

TL;DR I think Twitter Facebook et al do have something complex, but it is outside the hood rather than under it. (I guess they could have both.)

The "friending" system takes advantage of human's built-in reputation system. When I look at X's user page, it tells me that W, Y, and Z also follow/"friended" X. Then when I make my judgement of X, X leaches some amount of "free" "reputation points" from Z's "reputation". Of course, if W, Y, and Z all have bad reputations, that is reflected. Maybe W and Z have good reputations, but Y does not -- now I'm not sure what X's reputation should be like and need to look at X more closely.

Of course, this doesn't scale beyond a couple hundred people.