Zappos and L.L. Bean representatives responded within 24 hours to 100% of simple customer service inquiries posted on the Twitter social network, according to a test run for 45 days this spring by StellaService Inc., a vendor of customer service ratings.

The average response rate for the 25 large online retailers included in the test was 44%, which StellaService CEO Jordy Leiser says shows that not all e-commerce businesses are taking Twitter seriously as a customer service channel. The e-retailers that StellaService included in the test are the top 25 consumer-focused e-retailers listed in Internet Retailer’s Top 500 Guide, excluding Apple Inc., which does not have an official Twitter account, and Netflix Inc., which StellaService excluded because it does consider the movie rental and streaming service a traditional online retailer. The company had mystery shoppers tweet simple customer service questions to each retailer daily and measured the frequency and speed of the responses.

Other top responders on Twitter were Overstock.com and Dell.com, each of which responded to 98% of tweets within 24 hours, and Best Buy, which responded to 89%. Overstock.com Inc. is No. 27 in the Top 500 Guide. Dell Inc. is No. 5 and Best Buy Co. is No. 11. Zappos.com is a subsidiary of No. 1-ranked Amazon.com Inc. and L.L. Bean Inc. is No. 18.

In addition to responding to 100% of customer service tweets within 24 hours, Zappos.com also had the fastest average response time among e- retailers, responding to most tweets in less than an hour. Best Buy ranked second among the top five responders with an average response time of 1:47, followed by Overstock at 1:53. 

The number of U.S. Twitter users continues to grow, as does their activity on the social network, according to a Pew Internet & American Life Project report released today. 15% of U.S. adults in February survey say they use Twitter, up from 12% who said the same last August. Twitter users are also using the service more frequently than in the past. The February survey results say 8% of U.S. adults use Twitter on a typical day compared with 5% in August.

Broken out by demographics, the Pew report also says Twitter is most popular with younger, urban and black consumers. 26% of adults age 18-29 use Twitter; 28% of black, non-Hispanic adults do so; as do 19% of urban and 14% of suburban adults.