I have a Myspace and find that people mainly check it personel contact or event finding. I am not sure on the how effective it would be to market property. I think the one way that it could be effective is to promote a realator's blog or website.
The one drawback to that type of promotion is that you are mainly giving that info to people on your friend list. It is hard to reach a larger audience without looking like your adding "friends" just to spam them.
The one thing that I did just think of is that if a client has a Myspace a realtor could keep them infomed of anything intresting going on in their area. It would be an easy way to stay personally in touch with your a large number of clients.