Facebook really has nothing to worry about when it comes to Google+. I'm saying this even though I've not yet tried Google's (Nasdaq: GOOG) new product (you guessed it -- I haven't received an invite). But the reason I feel this way is very simple: the powerful force known as the network effect.

The ABCs of this network
The theory goes like this: The more users a particular "network" has, the more powerful it becomes. At a certain point, it reaches a critical mass that is hard to break. New users will join that particular network because that's where all the other users are, and existing users won't leave because all their friends are already on their network.

It's a very powerful effect, and it's the reason auction king eBay (Nasdaq: EBAY) has never come close to being deposed in the U.S. or any other places it's achieved critical mass -- despite an onslaught of well-financed competition. It's also the reason eBay was never able to knock Yahoo! (Nasdaq: YHOO) Auctions off its perch in Japan. Yahoo! got there first and reached critical mass first. Similarly, LinkedIn (NYSE: LNKD) has pretty much wrapped up the business-networking space.

I can hear you already: "Wait. Facebook mangled MySpace, so why can't Google+ flatten Facebook?" Good question. The answer is that MySpace never achieved critical mass. It was big with the younger crowd, but Facebook came along and appealed to all demographics and soon had the mass it needed.

So according to this theory, Facebook has nothing to worry about. And neither does Twitter, which I believe has also achieved critical mass in its own particular niche.

Unless, that is ...
Well, I guess I should mention the exception to this network-effect rule. A newcomer actually can disrupt the existing order of things if it offers enough improvement over the alternatives that the masses -- critical though they are -- simply can't resist switching over.

And this is where I may be wrong about Google+. As I mentioned, I've yet to try to the service. Who knows? Maybe it will be so magical and different that I change my mind. But I've read plenty of reviews, and so far the biggest selling point -- the ability to group your friends into "Circles" so you can share specific things with specific people -- already exists in Facebook. I use the feature, but not many people seem to know about it.

In the end, the biggest winners from Google's entry into the social-media scene are you and me and everyone else. The Circles feature will no doubt push Facebook to improve and highlight its own Groups feature -- and improve many other aspects of the service as well.

Bottom line
Mind you, I'm not saying Google+ will fail. It will probably flow down a slightly different path, and it may eventually find a niche we haven't thought about yet. But topple Facebook?  Not bloody likely, and you can share this article with all your FB peeps to let 'em know I said so.

Fool analyst Rex Moore tweets but is not a twerp. He runs a real-money, Rising Star portfolio based on his screens. Of the companies mentioned here, he owns shares of eBay. The Motley Fool owns shares of Yahoo! and Google. Motley Fool newsletter services have recommended buying shares of eBay, Yahoo!, and Google. Try any of our Foolish newsletter services free for 30 days. We Fools don't all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.