Facebook is tracking how happy people are in the United States.

Mother's Day was predictably upbeat, according to a Gross National Happiness index graph on the popular social networking service's blog on Tuesday, 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, intern Adam Kramer of the firm's data team said in an online post.

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," 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 sadest day in the two years measured by the index, according to Kramer.

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