I use both, although far from extensively. I don't remember the last time I logged into Myspace. For all I know, a bunch of hot Playboy bunnies all want to be my 'friend' and I'm missing out. However, it is quite handy for checking out new bands and hearing a few tracks to judge if they're any good without splashing out money on CDs/downloads.
I have mixed opinions about Facebook. I'm not sure I buy all the FBI/CIA stuff completely, but if it turned out to be true, I certainly wouldn't be surprised. All the BS applications it throws at you are a pain in the ass, but if you just keep it stripped down and simply use it to keep in touch with people, it can be very handy. It's helped me get back in touch with a few people I lost contact with, and it's great for organising parties and events. As for zombie wars and fun walls, thanks, but I'll pass.