From what celebrities are doing at any given minute to the little things that irk ordinary people, there's a lot to learn from Twitter.

Researchers at the University of Vermont used the social media service to learn about people's happiness and, through an analysis of billions of words used in millions of tweets, determined that societal happiness is on the decline.

Over a period of three years, scientists gathered 46 billion words found in tweets by 63 million users around the world and, with the help of a web site, determined the "happiness" of the 10,000 most common words in the English language.  "Laughter," for instance, got an 8.5 on a scale of one to nine, and "food" came in at 7.44. "Truck" was a better-than-average 5.48. "Greed" registered a 3.06; "terrorist," 1.30.

"We see that after a gradual upward trend that ran from January to April 2009, the overall time series has shown a gradual downward trend, accelerating somewhat over the first half of 2011," wrote the researchers, led by Peter Dodds, a scientist in the University of Vermont's department of mathematics and statistics.  The study is published online in PLoS ONE.

During 2009 and 2010, the happiest days, based on the number of positive words used, were Christmas Day, Christmas Eve, New Year's Eve, New Year's Day and other holidays.

"All of these observations are sensible, and reflect a strong (though not universal) degree of social synchrony," the authors wrote.

The non-annual event that was the most positive day was April 29, 2011, the day Prince William and Kate Middleton were married.  Tweets on this day were full of positive words such as "wedding," "beautiful" and "kiss."

"Negative days typically arise from unexpected societal trauma due for example to a natural disaster or death of a celebrity," according to the study.

The day the world learned of the death of Osama bin Laden ranked as the day of the lowest level of happiness, judging by the frequency of negative words like "dead," "death" and "killed."

The Chilean earthquake in February 2010 also ranked low in happiness, as did the March 2011 earthquake and tsunami in Japan and the October 2010 slew of storms in the U.S.  Declines in happiness were also evident after news of the U.S. economic bailout and the spread of the swine flu.

The researchers also found that happiness peaks over the weekend and dips on Mondays and Tuesdays.

But they also say while their study is an interesting look at how people feel on a given day or after a specific event, the findings don't necessarily reflect people's overall happiness.

"There is an important psychological distinction between an individual's current, experiential happiness and their longer term, reflective evaluation of their life," they wrote.