A quick question about monitoring workers

Hey folks, just a quick question that I wanted to ask you. I am becoming increasingly frustrated with the amount of time that I notice people are spending online while at work. This means that they are not actively making sales calls or doing what they are supposed to do.

My question is, I am actively considering using monitoring software on my computers to make sure that these is no ‘slacking off’ during work hours. Do not get me wrong, I am not a tyrant and I do not expect everyone to focus all the time, but if I know that people are actively using Facebook and eBay instead of working, I am starting to wonder why I should not just make it impossible for people to do this.

What do you guys think, do you have any experience with this? Are there any recommendations that you would like to make in this regard?