At risk of sounding like a cynical ass, my attitude toward social networking services has always been and will likely remain dismissive -- my impression is that it's little more than community building for dummies.
It takes time, effort and like interests to build a community like this and so many smaller communities -- a real personal investment made by many people. It's that grass-roots, personal investment that gives them their real sentimental value. These out-of-the-box mish mash communities appear, to me, to be nothing more than instant gratification match-making.
Inherently, a model with so broad an approach makes it the perfect business model and an ideal melting pot for . . . well, the kind of people this instant jack-in-the-box click-go-and-you're-in trendy stuff appeals to.
I don't mean to belittle people that use these services -- it takes all kinds to make the world go round. I'm just not personally compelled by these things. More sensation than substance.