The value of my house appears to be plummeting

I've always said that it's better to read about historic times than to live in them and here's proof. I plugged my address into Zillow.com to see what they had it valued at and it wasn't a pretty sight. It has gone from around $355k to $308k since July. The graph says it all.
For what it's worth, I had the house appraised a couple of years ago (back in the bubble), and the appraisal came close to matching Zillow's numbers.