One more stitch has frayed in the nation's social fabric with the latest society news: Many hostesses feel that it is no longer de rigueur to have an "extra man" at their dinner parties.

The extra man, it should be explained to those who equate the word "hostess" with "Twinkies," has been employed for years by people giving parties who would rather see the hollandaise sauce curdle than have more women than men at the table. By inviting an extra male guest, or by having a list of available men to be called at the last minute to sub for a no-show, a hostess could avoid the trauma of such asymmetric situations.

But whenever social customs change, someone is affected. In this case, it is the extra man, who after years on the dinner circuit suddenly finds that his company is no longer sought. Until now, whether he was a bachelor, widower, or divorced, the extra man was in demand at dinner parties, probably more so than if he were married. Hostesses flattered him shamelessly, he rarely had to cook for himself, and he never had to worry about finding a date.

It was a role that most single men relished, and with many it became a way of life. And if a man had particularly good looks, a fascinating job, or great wealth, he probably needed a social secretary to keep track of the extra-man offers that flowed in.

For many extra men, all this has changed. As people across town eat at dinner parties with no regard to the number of men vis-a'-vis women, the extra man is at home with a half-thawed macaroni-and-cheese dinner on the table, trying to make brilliant repartee with his cat. By day he haunts the mailbox and by evening sits by the phone waiting for invitations that never will be proffered. He soon realizes, in the words of one former extra man, that "All I was was a warm body in a dinner jacket."

As his withdrawal from the world of dinner parties deepens, the extra man may start to wander the streets of his neighborhood, black tie in hand, knocking on doors in search of an uneven dinner party. In a department store he'll gravitate instinctively to the dining-room furniture, where he will sit at a table and wait for other shoppers to join him. In church, when the story is read of Noah, the ark, and animals two-by-two, he may sniffle quietly.

Something must be done to combat extra-man withdrawal syndrome. Private charities and state aid can help, but it is up to the federal government to create a nationwide program to rehabilitate the extra man and help him reenter society. This is one aid program with which the Reagan Administration can have empathy: Funds can be solicited from the truly social.

There are two schools of thought on the rehabilitation of extra men. One view, held by a minority of hostesses studying the problem, is that the extra man should go cold turkey: no invitations, no canape's, no butlers, no raspberries. Particularly no raspberries. Most hostesses, however, favor the gradual withdrawal method.

Gradual withdrawal calls for reducing the number of invitations from several a week down to one a month; lowering dress requirements from black tie to dinner jacket to sports coat to Lacoste shirt; making the conversation progressively more boring; and switching caterers until you get down to fast-food carry-outs.

It is important to be on the watch for setbacks during treatment. The most common is hallucinations during which the extra man might think meat loaf is Beef Wellington or a House of Pancakes hostess, Betsy Bloomingdale.

As the withdrawal stage nears completion, it is time for the second phase of treatment. Part of this calls for teaching the extra man how to live on his own and includes a refresher course on washing dishes and buying foods other than eggs and beer, and eating with stainless steel utensils.

Perhaps more important,the extra man needs to be refreshed in the basics of dating: how to ask out a woman, what to do on a date other than attend a dinner party, and how far he can go on a first date. The thrill of the theater, even the gusto of a night of bowling are his to discover.