Is Facebook used or blocked in businesses?

It's hard to get good data on this, and I keep get wildly varying opinions. I haven't fully formed my own yet, but here's another data point:

"In contrast, 50 percent of respondents said that their company did not block access to [tag]Facebook[/tag], with eight percent specifying that the reason was fear of employee backlash." (from Sophos)

What do you think?