In some ways, we're all multiple personalities based on context.
I am a different person in the context of my family (where, silly me, I imagine myself to be king), in the context of my wife's friends (who think I'm with the CIA because of my mysterious trips out of the country), and in my various professional roles.
So I can completely justify being in multiple social networks with multiple purposes.
But with the explosion of online social networking, I face a multiple-personality problem. How many social networks are too many?