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.

In branding its important to make a quick yet distinquished impression that conveys your value.  Your avatar helps people start to establish trust with you.

All that being said - Peter Huboi has created for himself what I call the "best avatar on the web".  It has a clear image of his face but also includes some other elements which make you want to find out more.  Its brilliant and I never seem to tire of seeing it.  While Peter doesnt use it everywhere - its a refreshing change of pace.