R.I.P. Whitney Houston. Retweet for respect.

- @obeyMJ, Feb. 11

Many did as @obeyMJ asked, for one of the year's most retweeted tweets.

I Deeply Apologize To The McClain Family For Retweeting Their Address. It Was A Mistake. Please Leave The McClain's In Peace. Justice In Court.

- Spike Lee, March 29

The film director, incensed at the killing of teenager Trayvon Martin in Sanford, Fla., tweeted what he mistakenly thought was the address of alleged shooter George Zimmerman. It turned out to belong to a couple named David and Elaine McClain, with whom Lee settled out of court.

Higgs boson confirmed with 99% certainty: on.natgeo.com/Ny9BEe #CERN

- National Geographic, July 4

The Higgs boson, or "God particle," thought to pull together the standard model of the universe, is sort-of discovered at the CERN complex in Geneva, Switzerland.

Thanks to all my real fans and people who believe in me. I am now a living legend that's for sure.

- Usain St. Leo Bolt, Aug. 9

The humble Jamaican sprinter thanks his fans, and the world, after a fabulous double-repeat in the Olympic Games.

RIP Avalanna. i love you.

- Justin Bieber, Sept. 26

When 6-year-old fan Avalanna Routh died from brain cancer, Bieber posted his tribute, and it was retweeted more than 220,000 times.

If conditions are not safe on Wednesday for Trick or Treating, I will sign an Executive Order rescheduling #Halloween.

- Gov. Christie, Oct. 29

During the storm named Sandy, Christie used Twitter to inform, cajole, focus attention, tell of resources, announce executive orders, and encourage survivors.

Four more years.

- @BarackObama, Nov. 6

After a bruising election-year battle with opponent Mitt Romney, the reelected president celebrated victory with a now-famous image of Michelle embracing husband Barack.

word to big bird. #obama2012

- Questlove of Philly band The Roots, Nov. 7

Evidently pleased by the results of the election.

This election is a total sham and a travesty. We are not a democracy!

- Donald Trump, Nov. 6

Evidently displeased.

Dear friends, I am pleased to get in touch with you through Twitter. Thank you for your generous response. I bless all of you from my heart.

- Pope Benedict XVI, Dec. 12

The first-ever papal tweet from his new account, @Pontifex.