Washington Post Co. Earnings dropped 13.1 percent to $12.6 million in the second quarter of this year from $14.5 million in the same quarter a year ago, the company reported yesterday.

Earnings per share fell only from 92 cents to 90 cents, however, because the average number of shares outstanding declined from 15.7 million in last year's second quarter to 14 million this year.

Revenues for the quarter just ended rose 12 percent to $170.5 million from $152.3 million a year ago, the company said.

Because of the recession, both newspaper and magazine advertising linage fell, the report said. Nevertheless, newspaper division revenues rose 13 percent and those of the magazine division increased 8 percent because of higher advertising rates and subscription charges.

Broadcasting division revenues were also up 13 percent, largely reflecting increased advertising sales, the company said. Of the three divisions, only broadcasting was able to show an increase in net income from operations.

Increased expenses, particularly for newsprint, held down the newspaper division's earnings. Within the magazine division, Inside Sports, a monthly the company began publishing in March, lost $3.7 million, reinforcing the drop in earnings stemming from fewer ad pages. The domestic edition of Newsweek rand 830 pages of advertising in the quarter compared with 913 pages a year ago.

Another new venture, the Bear Island Paper Co., in which the Post Co. holds a 30 percent share, also had a loss in the quarter. The company began production late in 1979, and the Washington Post Co.'s share of second-quarter losses was $1.1 million, down from a $2.1 million loss in the first quarter of this year. The Bear Island loss reduced the company's income from affiliates to $397,000 for the quarter from 934,000 in the same quarter a year earlier.

Net income for the first half of 1980 was nearly double that of last year because of an accounting change in 1979. It rose to $16.4 million from $8.6 million last year. However, excluding the accounting change, which involved treatment of the costs of obtaining magazine subscriptions, earnings in the first half of 1979 were $22.1 million. On that basis, 1980 net fell 25.8 percent for the six months.

Macke Co. reported record sales for the third quarter ended June 30, while earnings dropped 17 percent to $1.3 million (43 cents a share) from $1.6 million (52 cents). Sales were $71.1 million, a 6 percent increase from $66.8 million.

Nine-month earnings were $3.5 million ($1.5) compared with $4.2 milllion ($1.37). Sales were $210.6 million, up 8 percent from $195.1 million.

Decreased profits in the vending and food service, caused by recession-related plant closings and layoffs, contributed to the third-quarter drop in net income, said Macke Chairman Meyer Gelfand. Profits in those areas dropped 19 percents, he said.

The restaurant division also added to the company's woes, Gelfand said.

Despite an improved profit performance in June, the Family Fish House restaurant chain had lower earnings this quarter than in the same period last year, Gelfand reported. On a year-to-date basis, the chain has had unfavorable operating results, the chairman said.