How has technology changed relationships?

Today I was sitting on the sofa next to my teenage son and we were watching some You Tube. Constantly people were popping up on his msn and facebook - including his girlfriend, who he sees at school and sometimes evenings and weekends, but no more than once a week. However I suspect they are in almost constant communication, or at least connected for communication if they want it.
How have these methods of communication changed the way we both make and conduct our relationships? The recent shocking case surrounding the dreadful abuse at the Little Teds nursery and the fact that the abusers had 'met' (although never actually in person!) via Facebook, and the use of their mobile phone, is a somewhat sobering reminder of how modern communications can have a negative angle (sorry, that sounds something of an understatement in this particular case!).
Before that case came to light, I had rather naively wondered if, as an interesting social experiment, it would be possible to use something like Facebook to build up a whole new 'network' of (previously completely unknown) friends from scratch. OMG!! How risky would that sound now.
What are others' experiences in this area? With all that is on offer out there online - including very specifically targeted 'dating' (or should we say, 'connecting people') websites, for almost every type of person/interest/status ... it makes a general run of the mill dating website seem a bit like the 'has been' we are all hoping to avoid when using them!! What do you think?