I believe both yahoo and google mix things up from time to time to see what sticks, nothing new, many have said the same thing. Today I observed a bounce back from yahoo. I had previously been ranked very high for my area, say #1 to #3 for various search terms. In recent weeks I fell to #4 to #10. I made a couple minor adjustments last week and today I popped back up
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Don't ignore Yahoo, it's a valuable source for visitors!