Having pondered this for a bit I think social networking websites are a lot more relevant when you don't have the skills (or perhaps inclination?) to start setting up your own blog, intergrating flickr/twitter into your own pages etc... etc... If you think about it you already "social network" a lot - you blog, you're active on discussions lists and bulletin boards, you've met and got to know ‘friends' entirely online - you just don't think of it like that cos you're not doing it via a single social networking website.
There is something a little weird about the whole Facebook hysteria... but having messed about with Ecademy and LinkedIn it's fun to suddenly find somewhere where all my non-techie friends are hanging out. I quite like having a space where I know only people i've actively connected to can see my stuff. And I like keeping up with what friends are up to without all the effort of actually interacting with them ;-)
But no, in the end there is nothing all that exciting to do there.