I never really considered Facebook and Myspace Professional social medias, but now that I think about it, a lot of businesses have pages and ads on Facebook. But do businessmen really post their professional portfolios on here too? I wouldn't consider that very professional because Facebook seems much more laid back or conversational (friends talking to friends, not businessmen promoting themselves). But maybe I just see it that way because that's the only way I use Facebook.
Linkedin and Plaxo seem much more professional to me. I think if I were an employer looking for someone to hire, I would first go to these sites before I checked out Facebook/Myspace. And I would consider it more professional if the person I was looking to hire told me to see his or her portfolio on Linkedin or Plaxo rather than Facebook or Myspace (but that's just my opinion).