The equivocating question mark in the title proves an appropriately dead giveaway. "The American Family: An Endangered Species?" -- NBC's three-hour report at 8 tonight on Channel 4 -- comes up not only short on answers but virtually bereft of life.

NBC president Fred Silverman has made a big deal out of promising more prime-time public affairs programming, but if the material is to be packaged with as little imagination and vitality an in "The American Family," audiences are not going to be attracted, and the gesture will be a mere public relations pantomime. "The American Family" should conceivably be of interest to every American family, but comparatively few will be able to hang on through all three of its gruelingly routinized, ritualized hours.

As it happens, ABC is the only network not devoting its entire primetime schedule tonight to one long program. CBS has slotted a three-hour TV movie, "The Incredible Journey of Doctor Meg Laurel," with Lindsay Wagner and Jane Wyman, opposite the virtually unbeatable ABC lineup of regular series (all of them, however, showing repeat episodes tonight). NBC is going to get the lamb's share of the audience, and the sad fact is, that's about what the network deserves.

Executive producer Stuart Schulberg and his staff appear to have gone at this project determined to abide by as many TV documentary formulas as possible. One might expect a nonfiction epic on the subject of changes in the American family structure and complexion; instead, a series of modular units is cranked out as if by computer or by the gang in Documentary 101 at Media U.

Among these units are fourteen filmed portaits of American families in various stages of schism or repair. Some of the vignettes are absorbing and convincingly microcosmic; others, however, deal with families so near one fringe or another as to make the lofty Louds of Pasadena -- that cross section of case studies featured on a public TV series earlier in the decade -- look positively next-doorsy.

In particular, a pair of lesbian mothers in Seattle, and their court battle to retain custody of their children by previous marriages, have already enjoyed sufficient media overexposure and contribute nothing of significance to his report. The cameras let into their home recorded such a sanctimoniously blissful impression of domestic life under homosexual parents (the kids play "My Country 'Tis of Thee" on their musical instruments and a young son prays in his pajamas) that it looks like a wholesome alternative to the heterosexual acrimony exposed earlier in the report.

A sequence on "cults" deals only with the Jonestown cult of Jim Jones and includes footage on NBC correspondent Don Harris interviewing Jones' mistress and her brother, who had come to Guyana hoping to lure her away from this so-called extended family and back to a real family. Three hours later, both Harris and the woman were dead, victims of the Guyana tragedy.

Interspersed with the portraits are "Profile" segments in which reporter Betty Rollin recites statistics (twice in the first half-hour announcing that 38 percent of new American marriages now end in divorce), "Point of View" segments featuring utterances on the state of the family by such dubiously enlightening authorities as Beverly Sills ("A family to me is just kind of a warm blanket of love") and Mort Sahl ("life without romance isn't worth living") and "Perspective" pills with Edwin Newman, as usual dour as Marley's Ghost, presiding over a sullen panel of theorizing shrinks.

Whatever the worth intentions behind "Family," NBC News has suceeded in pulling off a feat peculiar to network news departments -- saying entirely too much and yet appearing to say almost nothing on an overly broad topic. The report is particularly pallid when compared to such recent and more productively specific efforts as "But What About the Children?" on CBS and "Who Are the Debolts? And Where Did They Get 19 Kids?", acquired for telecast by ABC.

And though television is nothing if not a family medium which deals, usually in a misleading and frivolous way, with the American family every night of the week, there is not so much as a mention of TV and how it portrays or affects the American family in "The American Family." There is only a glimpse of one man sitting at home, beholding the face of David Brinkley on his TV set, which means NBC New is watching someone else watching NBC News.

Journalism like this gives escapism a good name -- and an audience of zillions.