The site, which boasts roughly 4,000 users, is based on the idea that following a weight loss plan is easier if the dieter has other people watching and evaluating what they're doing. "If somebody knows that someone else is looking at what they're doing, it brings a greater level of focus to their actions," said PEERtrainer CEO and Co-Founder Habib Wicks. "Even something as simple as having someone else point out a positive part of your day is a really powerful thing."

Currently, the site is monetized solely by Google AdSense units; membership to the site is free. But Wicks said that PEERtrainer is shopping around for "charter sponsors," to supplement ad income, and may be rolling out other premium account offerings in January.

Users of PEERtrainer are divided into groups of four people, with some groups based around location, interests, or diet type. Users then post journals about their weight loss progress and their diet and exercise schedules, and the members of their group comment and give advice and encouragement.

Habib said the company found that four-member groups are the sweet spot for the best user experience. "There's no personal accountability in large groups," he said. "In PEERtrainer the group of four is small enough that each individual person can get attention from the group, but it's also big enough that you have critical mass to have a group going--it just seems to work."

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