SAN FRANCISCO — By all rights, Twitter should be wildly popular with mainstream America.

World leaders, the pope, even Justin Bieber post updates there.

It tells us what's culturally relevant at any given moment, be it Beyonce's performance at the Super Bowl and the blackout in the New Orleans Superdome or Syfy channel's campy made-for-TV movie "Sharknado" about a tornado teeming with sharks.

But Twitter doesn't have nearly the following most people think it has.

Twitter will reveal for the first time in a year how many users it has when it makes public its stealth filing for an initial public offering as soon as this week.

As of last December, Twitter said it had 200 million people using the service, with most of those accounts overseas, nowhere near the following of Facebook with 1.2 billion users, or Yahoo, with 800 million. Recent reports estimate that Twitter users have increased to about 240 million.

In the U.S., its audience is far smaller. According to research firm ComScore Inc., Twitter had nearly 62.3 million unique visitors in August, up from 55.5 million in February.

The reason: Most people know what Twitter is, but many don't know how or why they should use it. Search for "I don't get Twitter" and Google spits out 5.7 billion results.

Now that it is on the verge of selling its stock to the public, Twitter has to prove to investors it can broaden its appeal or risk being pigeonholed as a niche service, analysts say.

"User base and growth have not been a problem to date, but Twitter is going to have to find a good system to reach tomorrow's Twitter users or eventually it will be a problem," said Max Wolff, chief economist and senior equity analyst at Greencrest Capital.

The Twitter IPO has captured the public imagination because Twitter has become such an integral part of popular culture.

It's a real-time spin room for politicians, an organizing tool for government protests, a wire service for breaking news and an online hangout to talk about live events and television shows.

Even its unusual conventions have saturated popular culture. Hashtags, a way of using a word to group tweets by subject, are on movie billboards and television ads. And "tweet" no longer just means the chirping of birds in the Oxford English Dictionary.

But Twitter executives have acknowledged for years that the service is not as welcoming as it should be.

"We have a lot of mainstream awareness, but mainstream relevancy is still a challenge," Twitter co-founder Jack Dorsey told students at Columbia University's Graduate School of Journalism in 2011.

Twitter has experimented with ways to make the service simpler and more personal as it faces rising competition from newer services such as Instagram and that resonate with younger audiences. It has also hired a former Google executive to spearhead overseas growth.

Still, it suffers from churn. People referred to as "Twitter quitters" get curious and try the service but then bail.

"Twitter is facing this dilemma as it struggles to get past a quarter billion users: Its growth is directly tied to what the public understands about what it is and how they can incorporate Twitter into their daily lives and get value out of it," said Brian Solis, an analyst with Altimeter Group. "But I don't even know if two or three Twitter employees would give you the same answer about what Twitter is."

Celebrities, politicians, media personalities, authors and other public figures have embraced Twitter, drawn to the ability to instantly publish and connect with followers. Some have turned their Twitter followings into digital gold. Tween heartthrob Bieber, for instance, has more than 45 million followers on the service.

But the torrential stream of updates, with the most recent at the top — not necessarily the most important — can overwhelm beginners or casual users.