Twitter is a great product, and millions of people have become addicted to using it for many hours per day (including me).

Investors have also gone bananas over the company, bidding its valuation up higher than $10 billion.

And advertisers have had very encouraging things to say about the power of Twitter as an advertising and customer-service channel.

But given the mind-boggling ubiquity of Twitter awareness--the little bird and @handles and quoted tweets are everywhere--it's easy to lose sight of something.

In the grand scheme, Twitter just isn't that big.

Oh, sure, in terms of sign-ups and monthly web visitors, Twitter is huge. Over 90 million people landed on Twitter.com last month. And the service blew past 500 million global accounts a while ago.

But the little-discussed secret about Twitter is that only a tiny fraction of those who have signed up for accounts on Twitter use it regularly.

And as far as regular usage is concerned, Twitter isn't even close to becoming a mainstream technology.

Don't believe it?

Here's what a recent Pew study found:

About one-in-ten Americans (13%) ever use Twitter or read Twitter messages. By comparison, more than half (54%) ever use other social networking sites, such as Facebook, Google Plus or LinkedIn.

Mainstream technologies and media have 30%-100% penetration, not 10%.

Facebook is a mainstream technology.

Google is a mainstream technology.

Smartphones are a mainstream technology.

Twitter is not a mainstream technology.

What Twitter is is a technology that almost everyone is aware of, thanks to its spectacular marketing and usage by dozens of high-profile celebrities to reach hundreds of millions of fans (here's looking at you, Bieber). But it's also a technology that only a small slice of the population regularly uses.

Now, from the perspective of those who are addicted to Twitter, this is very hard to believe. But the Pew study isn't an isolated freak finding.

For example, most Twitter addicts regard Twitter as the mainstream news medium for the digital age. But this, too, is a warped perception.

Another Pew study, from two years ago, asked Americans "where they got their news yesterday." Only 2% of those surveyed said they got it from Twitter. In this year's study, that percentage had increased only to 3%. Those are tiny numbers, especially for a technology that many people regard as ubiquitous.

Twitter is a great company and a great service, and it has a great mainstream brand.

But the service itself has not yet gone mainstream, at least not in terms of regular usage.

Those who keep arguing (on Twitter) that Twitter will soon vault past Facebook and crush television would do well to remember that.