The number of U.S. jobs posted online fell 13.2% year-over-year in May to almost 3.8 million, according to The Conference Board's online help-wanted ad measure. Compared to the prior month, however, online job ads rose 4%.

"May shows a slight recovery from the large April decline, but overall the number of online advertised vacancies has been on a downward trend for the past several months," said Gad Levanon, economist at The Conference Board. "The demand for labor will likely be sluggish this summer."