DES MOINES, Iowa - Sen. Chuck Grassley might be the only one surprised he's one of Congress' Top 10 Twitterers.

"I can't believe it," the Iowa Republican said in Cedar Rapids Thursday about a survey that found he was among the 10 most-followed members of Congress on Twitter.com, a free social-networking service that allows users to send and read other people's updates - or tweets - that are limited to 140 characters.

"I'm not doing as much as a I should," he said. "I love to do it and need to do more."

The survey by public relations firm Burson-Marsteller found that Grassley is one of eight Republicans in the 10 most-followed members of Congress. Sen. John McCain, House Minority Leader John Boehner and Sen. Jim DeMint are the top three. Only Democratic Sens. Claire McCaskill and Barbara Boxer were in that group.

Grassley, whose Twitter handle is @ChuckGrassley, has 20,323 people following him - receiving his tweets - and has posted 659 tweets.

By contrast, Sen. Tom Harkin has 767 followers of his @TomHarkin account and 682 people following @SenatorHarkin.