Facebook is an enormous threat to Google (Nasdaq: GOOG), that much is clear. But in the ongoing war between these two -- a war that’s being waged through big checks and sleek code -- Facebook is overmatched. Google’s moat is getting deeper and wider just as Facebook’s is running dry.

If that sounds a little nuts, it should. Facebook now hosts more than 500 million active users, a population two-thirds larger than the United States'. What’s more, estimates say the company will generate at least $1 billion in 2010 revenue, putting it in the neighborhood of top techies salesforce.com (NYSE: CRM), Akamai Technologies (Nasdaq: AKAM), and Netflix (Nasdaq: NFLX).

Yet, by itself, growth isn’t a competitive advantage. Crocs (Nasdaq: CROX) was a fast grower until it wasn’t. Only now, after the hype, has this one-time highflier settled into a niche. Facebook isn’t likely to suffer the same fate -- there are too many users -- but it also lacks a clear edge over alternatives.

Here are three reasons why Google investors should keep buying in the face of the Facebook threat.

Facebookers don’t need to be loyal
Take a look at the Social Web sometime. Alternatives are everywhere. Twitter is a massive and growing alternative to Facebook that already commands a $1 billion valuation. Google has Orkut, which has proven popular overseas, and of course News Corp. (Nasdaq: NWS) has MySpace. Netizens who frequent the Social Web tend to use more than one service to surf, search, and post.

We’ve known this for some time. It’s the reason services such as Ping.fm have gained in popularity: They allow users to update several networks at once. There’s even software to cater to our online multiplicities. Users have downloaded TweetDeck more than 15 million times since its introduction two years ago. A rival app, Seesmic Desktop, claims more than 3 million downloads. Both numbers make clear that netizens depend on not one but many social networks.

Code is portable
Apps are what make Facebook interesting as a business. At last count, there were 200 business applications built on the platform and many more categories with at least that many apps. Zynga has collected hundreds of millions in revenue by creating social games for Facebook. An IPO seems likely.

Trouble is, Zynga doesn’t need Facebook to be Zynga. Google has invested in the company, which took steps to host games elsewhere on the Web after Facebook blocked the company’s ability to send notifications and gift requests.

No one should be surprised to see Zynga cut the umbilical cord in this way. Facebook games such as FarmVille were built with the Web standards common to cloud computing, such as HTML and Java. Open standards make code portable, and developers are taking advantage.

Consider the smartphone market. Because of Wi-Fi and in-phone browsers, apps built for smartphones tend to mirror those built for the Web, allowing coders that write software for Apple’s (Nasdaq: AAPL) iPhone to repeat the process for Android, economically, even in the face of the iEmpire’s draconian do-it-our-way-or-not-at-all programming restrictions. That’s why retailers and wholesalers are reporting 200,000 new Android handsets sold each day. Platform lock-in is harder to achieve, even for Apple.

Email still beats every other form of digital communication
Social-media usage is undoubtedly on the rise. In January, Nielsen said the average consumer spent 5.5 hours on social media sites in December, up 82% year over year. In June, consumers spent 22% of their time online using social media.

Now consider what these stats mean. If 22% of time online is spent using social media, 78% of our hours online are spent doing something else. I’d wager email, instant messaging, and browsing.

Email remains the Web’s killer app, and Google is better at organizing and delivering mail than most. More than 100 million of us use Gmail; Facebook barely registers as an email provider, yet is trying to get better. Unless it actually does get better, Facebook has no shot at challenging Google.

Skeptics will argue that Gmail is flawed because it forces users to view messages as conversations. Please. If this really bothers you then fine, don’t use Gmail. But at least admit you’re nitpicking. An email trail is a digital conversation, why wouldn’t Google organize email to reflect reality?

More broadly, with Gmail, I’ve created a portal for accessing every method of business communication I use daily; email, instant messaging, and, yes, social networking, too:

Using Google services makes me productive; Facebook doesn’t. There’s no reason to consider Facebook a genuine threat to Google’s business until that changes, if it ever does.

Please take our Motley Poll then scroll down to the comments section to explain your vote.

Apple and Netflix are Motley Fool Stock Advisor selections. Akamai, Google, and salesforce.com are Motley Fool Rule Breakers recommendations. Google is also a Motley Fool Inside Value pick. Try any of our Foolish newsletter services free for 30 days.

Fool contributor Tim Beyers is a member of the Rule Breakers stock-picking team. He had stock and options positions in Apple and stock positions in Akamai and Google at the time of publication. Check out Tim's portfolio holdings and Foolish writings, or connect with him on Twitter as @milehighfool. The Motley Fool owns shares of Google and is also on Twitter as @TheMotleyFool. The Fool's disclosure policy likes pina coladas, but not getting caught in the rain.