Research In Motion Ltd., the BlackBerry maker that revolutionized the way workers communicate but has struggled to find a second act, saw investors abandon the company in droves Friday amid fresh concerns about its future.

Shares in RIM plunged 21% to their lowest level in five years as investors questioned whether its executives can pull the company out of its slump. RIM, which was worth more than $80 billion three years ago, ended the day worth less than $15 billion.

Demand...