I have had accounts on Facebook, Myspace, and Twitter.
I quit the lot of 'em in just a few days.
What a waste.
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.