Distinct social networks

With the number of people piling into Facebook and Twitter these days it is getting tougher to manage who is a friend, who isn't and how you define either. I still get jumpy about people who I don't know being able to call me a friend but am happy to call people who I only know online as friends - sometimes very good ones. At the moment I manage Linkedin and Facebook pretty strictly such that Linkedin is primarily business contacts and Facebook is people I actuall know - either business or otherwise. In Twitter I am more chilled about the definitions.