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I'm often asked 'What should the ideal website traffic sources pie chart be?' Overdependence on traffic from just search engines (and that really means Google) leaves a website vulnerable to any changes in the indexing algorithms.
The ideal is a balance, but the key is to monitor the changes to the chart's percentages over time. Referring sites % low? Do a reciprocal links thrust for a while.
Here's an article on what to look for in your Google Analytics.