Planning on firing off a short missive on Twitter or posting an update to your friends on Facebook from the office? Better check the rules of your workplace first.

According to a study commissioned by Robert Half Technology, an IT staffing company, 54% of US companies say they've banned workers from using social networking sites like Twitter, Facebook, LinkedIn and MySpace, while on the job. The study, released today, also found that 19% of companies allow social networking use only for business purposes, while 16% allow limited personal use.

Only 10% of the 1,400 CIOs interviewed said that their companies allow employees full access to social networks during work hours.

"Using social networking sites may divert employees' attention away from more pressing priorities, so it's understandable that some companies limit access," said Dave Willmer, executive director of Robert Half Technology, in a statement. "For some professions, however, these sites can be leveraged as effective business tools, which may be why about one in five companies allows their use for work-related purposes."

A study released last summer concluded that social networking use can hurt the bottom line.

Nucleus Research, an IT research company, reported in July that companies that allow employee productivity drops 1.5% in companies that allow full access Facebook in the workplace. That survey of 237 corporate employees also showed that 77% of workers who have a Facebook account use it during work hours.

Nucleus said the survey found that "some" employees use the social networking site as much as two hours a day at work. It did not say how many workers fit into that category, but did note that one in 33 workers surveyed use Facebook only while at work. And of those using Facebook at work, 87% said they had no clear business reason for accessing the network.