Google’s stock dropped 7% this morning – down to the $450s – after comScore sprung the lever on the trapdoor under its feet and reported that US paid-click growth in January was flat year-over-year and down 7.5% sequentially.

Google derives most of its revenue from paid-clicks.

ComScore said Yahoo’s paid-clicks were up 15% and cut 8 points off of Google’s market share.

The conclusion is Google has hit the economy’s headwinds, raising doubts about how it, basically an advertising company, will fare in a slowdown and whether it can meet first-quarter expectations.

In the fourth quarter Google admitted to a drop in paid-clicks but attributed it to new widgetry meant to cut the number of accidental clicks.

Google stock, near $750 not that long ago, is down 30 some odd percent since October and analysts are pulling in their price targets. UBS cut its number from $650 to $590 this morning; the average is down around $40-$50 to about $700.