Its important to present a representative image of yourself in all of your social network profiles.  People respond visually and make very quick decisions about you.  If your avatar is a cartoon its hard to take you seriously.   One caveat to that is - if you are not concerned about your brand then the avatar could be anything.  Sometimes you want to be anonymous or be known as something other than what you really are - thats playful and fun - but its not a personal brand.