NEW YORK (FORTUNE Small Business) - Want to find out what your customers are thinking for less than what a Madison Avenue consultant spends in a year on his lattes? Consider the case of Konstantin Guericke, the co-founder of LinkedIn, an online network that professionals can use to find new clients or pick up leads on jobs.

LinkedIn, based in Palo Alto, has 3.5 million registered users. To keep track of their needs, Guericke polls 20,000 to 30,000 of them each month using an online service called Zoomerang. The price: $599 a year.

Last March, Guericke was trying to figure out how to persuade more users to upgrade their LinkedIn accounts. Based on anecdotal feedback, he thought the answer might be allowing his customers to add photographs to their site profiles. To check his hunch, Guericke sent out a Zoomerang survey that asked users to choose their favorite from a list of possible new LinkedIn features, including photographs, discussion forums, and a search function that would allow them to reconnect with former colleagues.

About 80 percent of respondents chose the search function, compared with 50 percent or so who voted for photographs. LinkedIn added a search function in April, along with other enhancements prompted by Zoomerang surveys. The number of active LinkedIn users has doubled since then, Guericke reports.

Zoomerang is the market leader in a field that includes EZquestionnaire, KeySurvey and SurveyMonkey. All are easy, fast, and cheap compared with traditional telephone surveys. But unlike its competitors, Zoomerang owns a huge e-mail database that can be tailored (for an extra fee) to each client's survey needs.

Another plus: Zoomerang offers live help by phone in addition to the standard online tutorial. In contrast, SurveyMonkey doesn't even list its phone number in Portland, Ore. "We have a boatload of customers," says Ryan Finley, SurveyMonkey's 28-year-old founder. "If we did offer phone support, we'd have to build it into our pricing structure."

Online market research has its drawbacks. Phone surveys get higher response rates. And unless you're a whiz at designing surveys, you run the risk of asking the wrong questions, asking the right questions the wrong way, asking the wrong people, or analyzing the results poorly.

Guericke is undeterred by such obstacles. Two years ago he upgraded from Zoomerang's free service (available if you poll fewer than 100 people) because he was getting useful insights quickly. "Small companies don't have a lot of resources," he says. "Our advantage is to be nimble."

Feedback? Write tofsb_mail@timeinc.com

Also from FSB: Meet an entrepreneur who can survey 20 million consumers in two minutes.