Yahoo! ( YHOO ) investors may be asking, Who let the top dogs out? The exit of Yahoo!'s top European executive, announced Thursday, is the latest in a series of high-level executive departures that have dogged the company over the past year. Though the turnover doesn't appear alarming, and though other key executives are still in place, the changes add another dimension to the ongoing challenge facing Yahoo!: Building its revenue base and shoring up profits in a weakening market for the online advertising that is a cornerstone of the Internet company's business. In addition, the 86% decline of Yahoo!'s shares since the beginning of 2000 points up the challenge of holding on to old employees amid the falling value of stock options, and attracting new employees in an era in which equity-based compensation no longer has a lottery-jackpot type of glow. On Thursday, Yahoo!'s shares rose $2.81 to close at $31.31.