Facebook is now tracking how happy its users are with a Gross National Happiness (GNH) index.

The service, which has launched for the US and could easily be transferred to other countries, analyzes status updates for positive or negative words and compares them to a database of moods.

Mother's Day was predictably upbeat, according to a GNH index graph on the social networking service's blog, but for reasons unknown people's moods evidently sank days later. The pattern was the same for Father's Day.

Facebook began earlier this year gauging the nation's mood by tallying the numbers of positive and negative words used in status updates posted by users, Facebook data team intern Adam Kramer said.

In brief messages posted to social networking pages, Facebook users keep friends up to speed with thoughts and activities.

"Every day, through Facebook status updates, people share how they feel with those who matter most in their lives," Mr Kramer said. "These updates are tiny windows into how people are doing. Grouped together, these updates are indicative of how we are collectively feeling."

Facebook worked with psychologists and some of the more than 300 million members of its service to create collections of sunny and gloomy words indicative of whether people were having bright or dark days. The list of positive words includes "happy," "yay" and "awesome," while negative words include "sad," "doubt" and "tragic."

Holidays were consistently peak happiness days, and the GNH index shot up to double the average happiness reading the November day that the nation celebrated the election of President Barack Obama.

The saddest day in the GNH index was January 22, 2008, after the Asian stock market crashed and actor Heath Ledger died of an apparent accidental drug overdose at the age of 28. The death of King of Pop Michael Jackson on June 25 of this year marked the second saddest day in the two years measured by the index, according to Mr Kramer.

GNH results are currently based on updates in English but more languages may be added so indexes can be made for other nations.