This is truly a late comment but basically, developing a block policy is a stepped process :
1. Monitor: to see if it is detecting as it should.
2. Analyze results: what you are finding? - broken business process, inappropriate employee use, appropriate use that should be used as an exception. Write smart responses to help bucket your results including one that sends a message to employees (prior approval by HR and legal required).
3. Once you have warned employees, used your company's communication vehicles to let people know that a specific behavior will not longer be allowed and provided them with an appropriate way of doing the same thing, then.....and only then......should you put it on block.