A growing number of top-ranked U.S. colleges say they are finding objectionable material online that hurts the chances of prospective freshmen.

About a quarter of admissions officers at the nation's top 500 colleges have used websites such as Facebook and Google to vet applicants, according to an annual Kaplan Test Prep survey. Of those, more than one-third say they have found something that has hurt a student's chance of admission, up from 12% last year.

"We...