I’d like to see some research on how often profiles are completely removed or discarded, as having the facility to quickly break friendships, or delete information is different from actually doing it.

I agree that online relationships do lack the physical cues of offline relationships, but given that people only tend to have the same number of real ‘best’ friends on and offline (Around 15 I believe, I’ll try and find the source), most of the quick drops and adds are likely to be acquintances, and less meaningful

You could extend this point to include games like Second Life, where people are supposed to form relationships on a completely superficial level.

A friend of mine interviewed at Facebook, and during their tour they told her that their algorithms are able to predict when a couple on Facebook would break up, based on how often they looked at each others’ profiles and how much time they spent looking at the profiles of other opposite-sex (in the case of heterosexuals) Facebookers. It is a little creepy that Facebook may know better than you when you will breakup!