Athenahealth Inc. plunged, after hedge fund manager David Einhorn said investors have vastly overvalued the Watertown company, Bloomberg Businessweek reported. Einhorn, of Greenlight Capital, said the stock could fall 80 percent from record highs; it topped $200 in February. The shares are up nearly 50 percent in 12 months. Einhorn said athenahealth compares itself to cloud-technology companies like Amazon, which he calls a stretch, and he questioned its ability to compete against rivals like Epic Systems. Athenahealth did not immediately return a call from the Associated Press seeking comment.