Shares in Weight Watchers dropped by 19% on Friday after the company warned it was losing customers to free apps and other electronic slimming aids.
It expects to start 2014 with fewer members.
Weight Watchers cut its full-year profit forecast late on Thursday. It also said chief executive David Kirchoff was resigning.
The BBC reports the business operates by helping people manage weight loss through nutritional advice and products and motivational meetings.
It was founded 50 years ago and claims to have a million active members.
