LONDON - At these Olympics, Twitter is fire. Depending on how it's used, it can warm like a gas fireplace or scald like a blowtorch.

Twitter, the 140-character social media site, was barely a blip on the radar during the Beijing Olympics. Four years later, there are reportedly more than 500 million registered users worldwide, and the social media site has made a bigger impact in London than some Olympic teams.

In the past week: Two athletes were sent home from the Olympics for racist tweets; a current U.S. women's soccer player used it to chastise a former member of the program; a journalist had his account suspended after criticizing NBC's coverage, and countless viewers have used Twitter to complain about delayed television coverage -- and their inability to avoid Twitter long enough to avoid finding out results before events are aired in prime time.

Those are only some examples, though most athletes use Twitter as a safe way to be accessible.

Michael Phelps' record for most career Olympic medals has been the story of the Games so far, and it quickly became one of the most celebrated stories on Twitter. Late Tuesday night, after Phelps set the record, President Obama tweeted, "Congrats to Michael Phelps for breaking the all-time Olympic medal record. You've made your country proud. -bo"

Phelps wrote back: "Thank you Mr. President!! It's an honor representing the #USA !! The best country in the world!!"

But within a few days of arriving in London, two athletes embarrassed their countries via Twitter. The Swiss Olympic Committee sent home soccer player Michel Morganella for a racist tweet about South Koreans. The Greeks did the same with hurdler Voula Papachristou for a racist tweet about "Africans."

American hurdler Lolo Jones did not face overt punishment for her tweet, in the aftermath of the mass shooting in Aurora, Colo., that read, "USA Men's Archery lost the gold medal to Italy but that's OK, we are Americans...When's da Gun shooting competition?"

U.S. Olympic Committee spokesman Patrick Sandusky said the organization does not restrict athletes' tweets. "We just ask them to use their judgment," he said.

USOC chief executive Scott Blackmun believes the Americans' use of Olympic mentors has helped athletes. Since 2008, the organization has asked former Olympians to meet with new members of the team to inform them of issues they may face, including the damage that can be done by an ill-considered tweet or Facebook post. Gold medalists Jackie Joyner-Kersee and Dan O'Brien were among those who spoke to track and field athletes at the Olympic trials in June as part of the USOC's ambassador program.

Minnesota athletes at the Games have had fun with Twitter. Lindsey Berg, the former Gophers volleyball player, and the three members of the Minnesota Lynx, Lindsay Whalen, Seimone Augustus and Maya Moore, have provided running commentaries on their activities in London while promoting their sports. Marathoner Kara Goucher tweets pictures of her son, Colt. Rower Megan Kalmoe, who won a bronze medal on Wednesday, favors tweeting pictures of muscular male athletes. Timberwolves star Kevin Love has delighted in tweeting photos of his Team USA basketball teammates when they're asleep. Millionaires have never looked so much like kids at a slumber party.

U.S. fencer Mariel Zagunis, who carried the flag in the Opening Ceremony, said it boils down to a simple idea. "For me," she said, "it's just about being smart and respectful."

USA soccer goalie Hope Solo tweeted that former star Brandi Chastain should not be allowed to work as a TV commentator after Chastain criticized the team's play. But expressing differences of opinion is a common occurrence on Twitter; what proved shocking was the suspension of a journalist's account. Guy Adams, Los Angeles correspondent for the Independent, a British newspaper, ridiculed NBC's coverage early in the Games.

According to the Daily Telegraph, Twitter, which is partnering with NBC to cover the Olympics, notified NBC about Adams' criticisms and his inclusion of the work e-mail address of the NBC president of Olympics. Twitter suspended Adams' account, saying he broke the site's rule about including personal information such as e-mail addresses in tweets. Twitter later reinstated Adams' account and apologized.

The use of athletes' social media accounts to promote non-Olympic sponsors created controversy earlier this week. An International Olympic Committee rule specifically says athletes can't mention their own sponsors on social media during the Games. Several U.S. athletes protested -- on Twitter -- to object to the rule, which is meant to protect official sponsors who pay millions to be associated with the Olympics.

"Social media is so huge now and we're able to market ourselves in ways we haven't before, so it is definitely a different feeling having some restrictions on that," American distance runner Julie Culley said Wednesday. "But for the most part it's just been for being friendly. ... Instead of running into someone in the dining hall, you can get to know them on social media."