(FORTUNE Magazine) – Nobody can accuse real estate baron Mortimer Zuckerman, owner of U.S. News & World Report and the Atlantic, of being downbeat. Zuckerman on an expensive glass-and-chrome building he developed in Manhattan that remained empty for 18 months before nearly filling up with tenants: ''It is a great, great success. I think it is considered one of the most handsome buildings in the city.'' On rumors of big losses at U.S. News: ''The first six months of 1987 were the most profitable in the history of the magazine. Newsstand sales are up. Renewals are up. Everything is up.'' Property values surely are. In July, Zuckerman sold the Washington headquarters building of U.S. News to Shuwa Corp., a Japanese real estate firm, for approximately $80 million -- reportedly a record price for office space in the capital. He won't discuss the details of the deal, which includes a 25-year leaseback to the magazine, but U.S. News President Fred Drasner, once Zuckerman's attorney, says: ''It was a very good price, or we wouldn't have sold it.'' The product coming out of the U.S. News building is attracting customers as well. After a seemingly capricious circulation campaign, during which the price of the magazine was cut by one-fourth in 1985, only to be raised again ten months later, circulation rose 9.7% last year, to 2.3 million, the highest percentage circulation growth among the 20 biggest U.S. magazines. But Drasner says advertising revenues have been disappointing. About the only thing Zuckerman, 50, admits could be better is his personal life. ''I do regret it,'' he says. Never married, he split earlier this year from girlfriend Gloria Steinem, 53, editor of MS. magazine. ''That was the real thing for a long time,'' says an acquaintance of Steinem's. ''But they are very different people.'' She's big for causes; he's busy making money. Not that he wants to remain a solitaire. ''He has talked longingly about the idea of having children,'' says William Whitworth, the editor of the Atlantic. For now, though, Zuckerman's babies are his magazines.