For the past two years Google has been trying, without success, to develop a social-networking system to compete with Facebook, the social-networking powerhouse that keeps amassing an ever-bigger audience and siphoning away billions of dollars in online ad revenue. But last month, Google rolled out a new effort called Google+ (“Google Plus”), and guess what? This one has caught fire, attracting nearly 20 million users in just a few weeks, according to some estimates.

The funny thing is, the shot that Google fired at Facebook seems to have ricocheted and hit the wrong target. Because right now the biggest victim of Google+ appears to be Twitter, the microblogging service.

In fact, Google+ could be a death blow for Twitter, a company that after five years in business is still struggling to figure out a business model while coping with chronic management turmoil.

Twitter has been trying to sort itself out lately, but just as that effort begins, along comes Google+, a system that lets you do everything you can do on Twitter but is better in some important ways. Indeed, using Google+ makes you realize that Twitter doesn’t just need some little tweaks and fixes. The issue is that some of the fundamental things about Twitter just make no sense.

For one thing, when you use Google+ you’re not constrained by Twitter’s ridiculous 140-character limit. So instd of sqzng 2 much info in2 2 ltl space 4cing rdr 2 parse grble & get hdache, you can just read and write with full words like a normal human being. What a blast of innovation!

Also, when someone comments on your post, the comments show up right underneath your post. You know, like the way things happen when you talk to people in real life and they talk back to you. On Twitter, in contrast, you’re constantly seeing retweets but they’re scattered throughout your feed. Sometimes you get retweets even though you didn’t see the original tweet, and the whole thing feels messy and disoriented.

The biggest thing about Google+ is a feature called Circles, which lets you assign people into different clusters. Family members go into one circle, co-workers in another. So you can control which people see your posts. I can post photos of my kids to my Friends circle, but they won’t get shared with the people in my Work circle. When I post something that I want the world to see, I post it as “Public,” meaning anyone who follows me can see it.

Circles also improves the reading experience. Instead of getting everything mixed together, you can read feeds from each circle, apart from the others.

Sure, Twitter and Facebook both let you create lists, but it’s a pain in the neck compared with Google’s approach.

Google+ drew its first 10 million users in just 16 days compared with two years for Twitter and Facebook.

Social-media maven Robert Scoble has leapt all over Google+, and he recently raised some hackles when he wrote a blog post claiming that Google+ makes Twitter seem “boring,” and “old and crappy.” He added: “Google+ has beautiful photos and videos. Twitter? Just page after page of mind-numbing 140-character items. … Google+ is blowing Twitter away here.”

Dan Gillmor, another A-list social-media pundit, recently raved that he’d attracted more followers on Google+ in four weeks (more than 16,000) than he had on Twitter in four years.

This kind of enthusiasm helps explain a stunning chart that has been circulating recently comparing the early growth of Google+ with that of Twitter and Facebook.

Google+ drew its first 10 million users in just 16 days. Twitter and Facebook took more than two years to reach that milestone.

Before we get carried away, we should take a deep breath and remember that Twitter has 200 million users, 10 times what Google+ has. For that matter, Facebook has about 750 million users. It would take years for Google+ to grow that large.

But this is how disruption happens. Things start out small, sometimes as almost a joke. But they keep getting better, and more people start using them, and pretty soon they’ve conquered the world.

With social Web sites, what happens is that new sites seem cool at first, but then they get overcrowded and stale, and they start to fade away.

AOL once was hip, but today if you’re using an AOL email address you’re marked as hopelessly clueless.

MySpace once was the hot new thing, but then Facebook became the place where all the cool kids wanted to hang out and MySpace became a ghetto.

Now Facebook is the place where your mom and even your grandparents have accounts. Throw in all the creepy spammers, annoying ads, and cheesy Zynga games aimed at bored housewives in the Midwest, and suddenly the club doesn’t feel so hip and special anymore.

Then there’s the design. Instead of getting better over time, Facebook’s user interface seems to get worse. Its front page is cluttered and ugly, while Google+ has a clean, minimalistic user interface, with lots of white space. Google+ feels fresh and modern, while Facebook seems low-rent and trashy.

A bigger problem for Facebook is that people really don’t like the company. A study last year showed it ranked next to airlines and cable companies in terms of customer satisfaction. Facebook brought this on itself, with too many sneaky privacy changes, too much hubris and doublespeak.

How soon until using Facebook seems as lame as having an AOL email address? As far as Google is concerned, not soon enough.