Google's (NASDAQ:GOOG) shares slid 3% Thursday as investors worried that the search-engine outfit may not be invincible after all.

The anxiety cropped up after Google reiterated a warning that seems fairly reasonable: It simply cannot sustain its incredible revenue growth rate. More specifically, it noted that top-line growth in the fourth quarter may not be on pace with that of the third. The statement, although made relatively low-key in a regulatory filing with the Securities and Exchange Commission, created quite a stir.

Of course, investors don't have much reason to fret at the moment. Google continues to innovate and leads in a variety of areas. Yahoo! (NASDAQ:YHOO), for instance, is still playing catch-up with Gmail and trailing Google's lead in desktop search.

At the same time, Google's momentum may slow further as the competitive environment heats up. A recent United Press International story reported that rivals Yahoo! and Microsoft (NASDAQ:MSFT) are successfully mimicking the basic concept behind Google's search technology. Yahoo! lately managed to secure an extension to place ads on MSN sites through June 2006, but after that date, it seems another rather powerful competitor will vie with Google in the business of ad-sponsored search engine listings.

Further, while sponsored search remains the killer Internet business model of today, things change quickly in cyberspace. Yahoo! seems to be gravitating toward original or exclusive content that leverages the increasing use of broadband in U.S. homes. Amazon (NASDAQ:AMZN), meanwhile, is more actively experimenting with original broadband content. It is unclear whether these initiatives will emerge as successful revenue generators, but if they do, a major shift is surely forthcoming.

Rapid innovation remains one of the hallmarks of the Internet. Google has led the way, but its rivals are by no means standing still.

Fool contributor Brian Gorman is a freelance writer in Chicago. He does not own shares of any companies mentioned in this article.