Yes. Allowing a reasonable amount of personal use of school IT is good for work-life balance in my view. As professionals, staff are expected to manage that time so that their use does not interfere with their duties.
We do have Smoothwall in place so use is logged, and I am militant about staff locking workstations to prevent kids getting to it.
Also worth pointing out that in my last school, which was a secondary school, we allowed Facebook for students too, subject to the same expectations. It was actually very useful in gathering evidence of cyberbullying that would otherwise have happened at home where we couldn't monitor it.