It’s tough to imagine that anything could unseat Google (Nasdaq: GOOG) as the king of all-things-Internet.

After all, not only does the company dominate the search engine market, but it also keeps people in the Google circle by constantly adding new tools and features to its repertoire. The Google Docs office software suite, online picture-filing-cabinet Picasa and its mapping software are three of the free services Google has provided to users in an effort to pull them deeper into the fold.

As is so often the case, however, time and success appear to have finally caught up with Google, and the features that made it great in the early part of its existence aren’t keeping up with the competition anymore. Ironically, the very competition Google was whipping just a few years ago is catching up — with a vengeance — on three specific fronts. 

1. Google is losing key executives
The loss of one key executive to a competitor is annoying, but when two top executives defect to direct competition, then it’s a headache. When the number reaches three, there’s an obvious pattern setting up.

Google’s highest-profile loss in recent memory is the exit of Marissa Mayer. She spent 13 years with Google and served as vice president of local, maps and location services before she was named as the new CEO of Yahoo! (Nasdaq: YHOO) in July.

Though the raw proprietary technology Mayer helped design at Google will be staying with her old employer, her knowledge and experience will give Yahoo! some amazing insight on how to compete successfully in the next evolution of Web usage. Mayer has made no bones about the fact that Yahoo! is embracing the importance of mobile Web and local Web. Indeed, she’s so intent on taking the company in that direction, she recruited a new chief operating officer with relevant experience. It’s Henrique de Castro, who was acting as the president of global media, mobile and platforms for… you guessed it, Google.

And this is still not the entire list of strong leaders Google has lost.

The current CEO leading the surprising turnaround at AOL (NYSE: AOL) is Tim Armstrong, the former president of Google’s Americas operation. Apple (Nasdaq: AAPL) has also scooped up several key Google employees to fix some glaring problems with the new iPhone’s maps application.

But the talent-bleed is only half of the problem festering for Google.  

2. Google’s strategies are being applied elsewhere
Google’s amazing growth has been fueled by an almost formulaic approach to building its brand.

At its core, Google thrives on projects, or the development of incremental ideas or revamps — sometimes through acquisitions — that are perfected or punted within a set period of time. If a project works well and drives revenue, then it’s kept. If it doesn’t work well, then it’s dropped and the team that was working on it moves on to other assignments. Either way, no Google employee has a chance to ever get in a professional rut (in a good way).

The problem with such an approach is that it’s easy for other companies to adopt it and generate the same kind of success as Google has had with it. And sure enough, Mayer and Armstrong are using the same project-deployment techniques to rebuild Yahoo!’s and AOL’s websites as destinations chockfull of useful tools and information. 

As an example, Mayer is leading the charge to revamp the Yahoo! homepage, ultimately in an effort to increase the website’s traffic. The so-called “Project Homerun” is clearly big. Once it’s implemented and put into motion, traffic is likely to ramp up, but the team currently working on the project will have no need to keep working on the task. It will likely move on to other tasks.

Armstrong has also applied the one-time-overhaul approach. One of his more prolific projects is nicknamed “Project Devil,” which is designed to improve the effectiveness of AOL’s display ads. 

In simplest terms, Google is no longer going to be the only Internet company to tout new features constantly. 

3. Google is now a behemoth
It may be unfair, but the bigger a company gets, the tougher it is to keep it growing. Mathematically, the bigger a company gets, the less impressive its percentage-growth rate becomes — even if, in absolute terms, growth remains strong. That’s where Google may be right now. The easy money and easy market penetration are behind it, and now that its competition has started to catch up, Google’s competitive edge is getting relatively dull.

Throw in the fact that its ad-revenue growth rate slowed during the third quarter while Facebook (Nasdaq: FB) made huge strides on the same front, and it isn’t hard to wonder whether Google has already peaked.

Risks to Consider: When it comes to the Web, companies are just one app or service away from ruling the world or falling completely out of favor. Though unlikely, if for some reason Yahoo! whiffs on its effort to become a major mobile player, or Google somehow breaks into telecom-service space, then it could change the entire Web landscape in an instant.

Action to Take –> Just to be fair, Google is apt to remain the king of search for a long, long time. This doesn’t mean its future growth will be as strong as its historical growth, however, now that AOL and Yahoo! are being led by some of the very same people that put Google on the map, using the very same project-management approach. 

Between these two companies, Yahoo! may be the better stock pick just because it’s starting with a much stronger presence in the marketplace. Given the third-quarter’s solid income numbers, shares could be worth as much as 40% more within 12 months. The company beat estimates and reported earnings of 35 cents per shares in the third quarter, 10 cents more than analyst expectations, so it looks like Mayer is off to a great start. The forward-looking price-to-earnings (P/E) ratio of 14.3 is fair as it is, but if Yahoo! keeps beating analyst projections like that, then the forward P/E ratio could likely turn into a single-digit figure. 

Meanwhile, Google has disappointed. The search engine giant actually missed earnings expectations of $10.65 per share for the third quarter, turning in a only $9.03 in per-share earnings. It was the second miss in a year.