As Twitter grows up, one of the best examples of its becoming mainstream is how widely and quickly real-life celebrities have latched onto the service, pushing Twitter's early heroes -- Internet-famous-types and tech journalists -- aside.

Consider the differences in the most popular Twitter users today and just 15 months ago. This time last January, Twitter's two most famous human users were Robert Scoble, a tech blogger best known for his years at Microsoft; and iJustine, a cute girl in her mid-20s famous for strapping a Web cam to her head and fawning over Apple gadgets. According to a Web cache of Twitterholic, a Twitter user-tracking service, Scoble had 6,900 followers on Jan. 18, 2008, and iJustine had 6,700.

But fast-forward a little more than a year and Twitter's top users have more than 150 times the number of followers and millions more dollars in the bank. Actor and wannabe tech entrepreneur Ashton Kutcher got his 1 millionth follower today, having beaten an account owned by CNN for the milestone. Wednesday, Kutcher blew past third-place Britney Spears and her 950,000+ followers.

The fact that a race to 1 million followers existed at all is nothing short of amazing for Twitter, a barely three-year-old messaging/microblogging service.

Today, Kutcher and Twitter CEO Evan Williams will appear on the Oprah Winfrey show, where Oprah will send her first tweet as @Oprah. As of Friday morning, she had 63,000 followers without sending a single toot. Within weeks, Oprah could be one of Twitter's top users.

Of course, Williams and Twitter's other founders, Biz Stone and Jack Dorsey, had a lot to do with this. It wasn't until recently that Twitter added a new feature to the sign-up process, which recommended several Twitter feeds for new users to follow -- so they get the hang of the service. This, coupled with Twitter's rapid growth over the last three months, boosted those suggested accounts considerably.

Twitter includes many real-world celebrities on its list of suggestions, such as Phoenix Suns star Shaquille O'Neal, one of the earlier -- and one of the best -- celebrity Tweeters; and some Internet-celeb-types, too, such as iJustine. (And even our feed.) Shaq now has 660,000 followers, up from 5,500 last November; iJustine now has 435,000.

"I'm happy for Evan, Biz, and Jack... they played it exactly right," says Jason Calacanis, long-time Internet entrepreneur/celebrity-type and former Big Man On Twitter. "They courted early adopters and now are courting the celebs to get the mainstream -- that's perfect strategy."

Last month, Calacanis publicly offered the company $250,000 for a two-year slot on the suggested list. Twitter hasn't accepted yet. But that's one of many easy ways that Twitter could generate significant revenue -- when it decides to.

Update: This chart and story text previously incorrectly referred to the Jan. 18, 2008 stats as Jan. 18, 2009 stats. This has been updated; we apologize for the error. It does not change the thesis.