U.S. service companies grew at a slower pace in April, as business activity generally showed a levelling off.The Institute for Supply Management, an association of purchasing managers, reported Friday that its service index fell to 55.5% last month, down from 56.1% in March. Any reading above 50 signals growth.Companies surveyed for the index were mostly optimistic about overall business conditions but expressed continuing concern about finding qualified employees, the ISM said.The service sector, which is where most Americans work, has been expanding for 111 straight months, according to the ISM index.The Associated Press