Two months later, he had millions of followers (the count now sits at 3.1 million) and a book deal with HarperCollins. That book hit No. 1 on the New York Times bestseller list and led to a short-lived CBS sitcom, "$#*! My Dad Says," starring William Shatner.

Halpern still tweets out his dad's best moments. His second book, "I Suck at Girls," was published last May.

Sohaib Athar

Athar was a 33-year-old "IT consultant taking a break from the rat race by hiding in the mountains with his laptops," according to his Twitter profile. That spot in the mountains was in Abbottabad, Pakistan, and on May 2, 2011, he tweeted about a curiosity.

"Helicopter hovering over Abbottabad at 1AM (is rare event)," he wrote.

Little did he know that, over the course of the next few hours, he'd become possibly the world's first person to unknowingly report on the death of terrorist Osama bin Laden at the hands of a U.S. Navy SEAL team.

All of a sudden, news outlets from around the world were scrambling for interviews with him. His modest 750 Twitter followers ballooned to more than 105,000 (they've since settled back to about 64,000).

He continues to tweet from Abbottabad but has traveled extensively, including accepting an invitation to tell his story at last year's South by Southwest Interactive festival in Austin, Texas.

Anthony Weiner

He claimed that he was hacked, and at first, some of us believed him. After all, could a U.S. congressman be so clueless?

Turns out ...

In June 2011, then-New York Rep. Weiner resigned after someone used his Twitter account to send suggestive photos to some of his female followers. At first, he lied, saying he'd been hacked. But after a couple of frantic days, Weiner fessed up that he had been having inappropriate online relationships with women he met through social networking sites.

He, perhaps wisely, also quit Twitter for a while. His first post since the scandal was in November, when he tweeted about Hurricane Sandy. The potential New York mayoral candidate's most recent tweet, from February, suggests that he still may not have gotten the hang of the whole Twitter thing.

"Llp@," it reads.

Paraskevi Papachristou

A Greek triple jumper, Papachristou was hours away from realizing her dream of becoming an Olympian. Then, on her way to last year's London Games, she tweeted a joke:

"With so many Africans in Greece, the mosquitoes from the West Nile will at least be eating some homemade food."

Maybe it was supposed to be some kind of play on words. But it was quickly denounced as racially insensitive, or downright racist, by Twitter users.

Greece's Olympic committee condemned the tweet and ruled that she would not be allowed to participate in the games.

For what it's worth, Papachristou's last tweet, from July 25, expressed "heartfelt apologies" for the joke, saying she "could never believe in discrimination between human beings and races."

Ashton Kutcher

Sure, Kutcher was already a TV and movie star when Twitter started up. But he became the first Twitter celebrity after joining in January 2009, when the site was getting ready to make the leap from tech-savvy coffeehouse to household name.

He got tons of publicity for becoming the site's first user with 1 million followers -- a distinction he won after winning a race to seven figures with some news network called CNN. He also became a savvy investor in tech startups.

But perhaps more importantly than sheer numbers -- he's now 23rd on the site's popularity list, with almost 14 million followers -- Kutcher seemed to be the first celebrity who understood the benefits of using Twitter to interact with fans.