This video explains it better than I can:
http://www.securitytube.net/video/1084 Changing Threats To Privacy From Tia To Google (Blackhat 2010) by Moxie Marlinspike
The relevant part:
"I think if you look at the way that people tend to organise in groups and communities, there are often informal communications networks that bind them together, that allow them to communicate, make plans, coordinate activities. If you introduce something like the GSM cellular network to this group, and if I start using it, I am subject to something that is very well known called the No Network Effect. If I am the only one with a cell phone it's really not worth very much. The value of the network is in the number of people that are connected to it and that if I'm the only one I can't really communicate with anyone.
However if I somehow manage to get everyone to start using my communications network it becomes very effective and very valuable. But there is an interesting side effect, which is that the old informal mechanisms people use to communicate and to collaborate disappear, that they are destroyed by the introduction of technology. The technology actually changes the social fabric of how people communicate and coordinate. Mobile phones, there are many obvious examples. People used to make plans, they would say: "I'll meet you on this street corner at this time on this day and, you know, we'll do something" and now people say "I'll call you when I'm getting off work" or "I'll text you" and if you don't have a mobile phone you can't really participate in this type of organisation and you begin to find yourself kind of alone. Because if I now make a choice not to be a part of this cellular network, there is sort of an interesting thing where once again I am subject to the no network effect. The network that used to exist, the informal communications channels, has been destroyed.
So yes, I made a choice to have a mobile phone, but what kind of choice did I make? I think this is sort of an interesting phenomenon. What happens is a choice is introduced; it starts as a very simple choice: the choice of whether or not to have a mobile phone, a simple piece of technology. But slowly things happen to expand the scope of that choice until eventually it's so big as to encompass not just whether you have a mobile phone or not but whether you want to be a part of society. In some ways the choice to have a mobile phone today has become not necessarily just whether you have a piece of consumer electronics in your pocket but whether or not you are even a part of society, and that's a much bigger choice. Maybe not one that we should have to make, or at least maybe one that isn't really a choice at all."
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There's some irony here now that Moxie has teamed up with Facebook to work on WhatsApp.