Twitter has been a tremendous tool for fans, journalists, coaches and athletes to keep in touch with the people who they follow and follow them.

For some, it's the community the social media platform provides during games. For others, it's strictly a news service. No matter the function, it seems almost everyone in sports is on Twitter today.

That is, except for a few notable holdouts that the sports world loves to follow.

This topic has been much discussed in my circles in recent days, as Kobe Bryant, who doesn't have a personal account on Twitter, tempted us by taking over Nike Basketball's Twitter handle for the past couple of days.

So I asked my Twitter followers: Who is missing?

The most popular answers, likely because of their appeal to both men and women, were Tom Brady and Derek Jeter. And sex appeal doesn't hurt someone like Maria Sharapova, who has 9.1 million fans on Facebook, but doesn't tweet.

After that, I noticed there were two categories of people that sports fans wanted to see. Popular athletes and guys who they know would just shake it up.

In the popular athlete category, there were requests for Peyton Manning, Michael Jordan and Dale Earnhardt Jr.

It appears there were even more requests for those in the powder-keg category.

You know the names: Bobby Knight, Charles Barkley -- even a couple of nods for Jon Gruden.

For sports figures who have stayed out of the Twitter game, those who manage their careers say there's little temptation.

Despite all those who extoll the virtues of the power of social media, Carlos Fleming, the marketing agent of Carolina Panthers quarterback Cam Newton, says "the downside of Twitter outweighs the upside."

Fleming says Twitter means the athlete chooses to "always be on the microphone," and one mistake could overshadow years of good.

Many people want to see basketball legend and current Bobcats owner Michael Jordan on Twitter, so much so that imposters have seen their follower count skyrocket before being found out. But Jordan's manager, Estee Portnoy, says nothing is imminent.

"I never say never," she said. "But he doesn't do it now nor does he have immediate plans to join."

Same for Andrew Luck, who unlike fellow rookies Robert Griffin III and Russell Wilson, is not on Twitter, and simply has no desire to try it out, according to his agent, Will Wilson.

"He may reconsider in the future," Wilson said.

Perhaps when more big endorsement deals offer more money to tie in tweeting?

Maybe not. Phil Mickelson is among the highest-paid sports endorsers in the world, the second-highest-grossing athlete according to Sports Illustrated with $57 million in endorsements, and he isn't on the site.

Said T.R. Reinman, spokesman for Gaylord Sports Management, which represents Mickelson: "To date, Phil has shown no interest in a Twitter account."

That doesn't mean the current holdouts can't change their minds. Sharapova will make her debut on Twitter in January, says her agent, Max Eisenbud.

"When re-evaluating how she reaches her fans, we understood that there are people on Twitter who are not on Facebook, and we want them to be able to keep up with Maria in the same way," Eisenbud said.

Eisenbud isn't all-in on Twitter himself, having seen the ugly underbelly of the beast.

"There are some things athletes are doing on there that I think is a disservice and a turnoff to their fans," Eisenbud said. "Be out there, interact, but some of these personal tweet conversations between other people that nobody else can understand doesn't make much sense."

Here's the full list (gathered on my own Twitter feed) of the most-requested sports figures who currently are not on Twitter (in order):

1. Tom Brady

2. Derek Jeter

3. Michael Jordan

4. Kobe Bryant

5. Charles Barkley

6. Dale Earnhardt Jr.

7. Peyton Manning

8. Phil Mickelson

9. Cam Newton

10. Bobby Knight

11. Maria Sharapova

12. Tony Stewart

13. Roger Federer

14. Jon Gruden

15. Rex Ryan