The other man’s grass is always greener.

And the other man’s Facebook page frequently leaves visitors green with envy, according to German researchers who recently released a study entitled “Envy on Facebook: A hidden threat to users’ life satisfaction?”

Social scientists at Berlin’s Humboldt University and Darmstadt’s Technical University document a reduced “sense of personal happiness” among many people who see Facebook-posted images and written accounts of other folks’ fun vacations, heartwarming family moments and even exciting nights out on the town.

A majority of the 357 people surveyed, predominately German university students, even said they felt jealous when viewing Facebook pages that have drawn more “likes” than their own.

The researchers wrote: “Our findings signal that users frequently perceive Facebook as a stressful environment, which may, in the long run, endanger platform sustainability.”

Behold another troubling side-effect from the strange new world of social media. In that often-illusory cyber realm, a Notre Dame football star was a dupe and/or duper in the fabrication of a story about a girlfriend who died, when in reality she did not exist.

And that’s only one of countless instances in which a wide range of misinformation and hoaxes spread through assorted personal websites at staggering speed. Thus, a technological advance in self-expression fuels widespread diminishment of self-esteem.

Of course, there’s nothing new about wondering why somebody else seems to have it better than you — on or off the Internet.

But just as you can’t judge a book by its cover, you can’t judge a person’s “life satisfaction” by a Facebook page — including your own.