Facebook is expected to pull in approximately $100 million in revenue this year, but that number isn’t likely to impress Australian employers who are expected to lose as much as $4 billion to declining productivity each year because their employees log on to the social networking site while at work.

That’s according to a recent study released by Internet security company SurfControl and reported on by Reuters on Monday.

The news agency reports that SurfControl Chairman Richard Cullen told Australian radio that the site is “so interactive that people just get addicted to watching their Facebook groups all the time.”

“People love being there and telling people what they are doing right now, what their thoughts are right at this second," he said.

Facebook has seen tremendous growth in Australia over the past several months, according to Hitwise. Visits to the site by Australians have jumped significantly since the Palo Alto, California-based social network launched a platform for third-party application developers in late May. Hitwise reports that by the end of July, Facebook had become one of the top 20 websites visited by Australians across all categories.

One can only guess how many billions of dollars are lost due to a drop in worker productivity as a result of visits to MySpace, the social network with the highest market share in Australia at 2.2 percent.

Reuters said SurfControl’s figures are based on an typical Facebook user who earns the average Australian national salary and spends one hour online each day.

Upon learning of the study’s findings, employers may be tempted to block employees from logging on to the site. In fact, the U.S. military banned the use of MySpace, hi5, BlackPlanet, and other social networks on Department of Defense networks earlier this summer.

Still, employers might want to take note of the case of international law firm Allen & Overy, which attempted to block its employees from Facebook earlier this year. Legal Week reports that the company had to retract its stance against the social network after workers complained that the site was an important tool they used for business purposes.