Social networking sites like Facebook and MySpace aren’t very popular in the corporate world, according to a study by Barracuda Networks.

Analyzing data from businesses using its Web filtering appliance, the company found that 44 percent block MySpace while 26 percent block Facebook. More than 50 percent block one of those sites or both.

“It was interesting to us to see such a significant backlash in the corporate environment, with 50 percent blocking the social networking sites. And that number will go higher,” Dean Drako, Barracuda president and CEO, said in an interview. “Customers that weren’t blocking but were monitoring social networking sites … a significant percentage expect they’ll be blocking those sites soon.”

Customers said they were concerned about the sites being a productivity drain, Drako said. They also were worried about offensive content on MySpace.

A separate survey of 228 IT professionals by Barracuda showed that the top reason businesses restrict employee Web surfing overall is to block viruses or spyware. Productivity was the second biggest reason.