Myspace is good if you are in a band. Very nice to get your music up on the net cheaply and get some exposure. Other than that I don't see it has much Value.
Facebook works well for people who want to keep up with family members though I am leery of putting too much info up. So I don't But Facebook works best for famous people to get info out to their fans.
Twitter is the on I use the most BUT I use it to drive traffic to my EBay listings. Its amazing how many people have hashtag (keyword) alert programs/apps for twitter on their smartphones. I've sold many an eBay item because of my eBay tweets. Also much of the "Tea Party" movement was coordinated and arranged on Twitter.
As for myself, an old "geezer" retired guy, it's a waste of time as far as I'm concerned.
Not to mention a possible invasion of my privacy. imho.