Network proxies are used for a number of reasons. There is the type that many organizations use to limit what their employees (or students, etc.) have access to, which is the reason you and your coworkers do not have access to Facebook or Gmail. They can also speed up access to websites and reduce the amount of internet traffic by caching or storing copies of pages that are frequently accessed. Normally, the proxy software is configured so that only users inside the network have access to it. In this case, however, it is misconfigured so that everyone inside and outside the network can access it. The effect is that anyone, anywhere in the world, can appear to be a user on your company's network. People often access this type or proxy for privacy reasons, but they are most often used as a gateway to send spam, to launch network attacks, to facilitate viruses and trojan horse attacks, and, sadly, to vandalize Wikipedia articles, which is the reason we automatically block them.

But to get back to your questions: I have no way of knowing when the IP address began acting as an open proxy, but it appears that it was detected and blocked by an automated process here on October 22, 2011. And, no, most organizations do not employ open proxies due to the security concerns I mentioned above. It appears that the IP address has not been included in any anti-spam blacklists yet, but if you run a Google search for it, you will see that there are some indications of trouble. Most network admins would be very glad to hear about this sort of issue before the IP address appears on blacklists gets blocked by their own or other internet providers.

I hope this clears it up for you, but feel free to ask more questions if not. Cheers ​— ​